Performance and connectivity

Secure connection failed and Firefox did not connect

If a website requires a secure connection that cannot be established, Firefox will not connect and will show you an error page. Learn more.

How to troubleshoot time related errors on secure websites

Learn why you might see time-related errors on HTTPS websites and how you can troubleshoot these errors by correcting your system clock.

Website will not allow Firefox to display the page if another site has embedded it

This page explains the error: "Website will not allow Firefox to display the page if another site has embedded it"

Connection settings in Firefox

Your organization or Internet Service Provider may offer or require you to use a proxy to connect to the Internet. Learn more.

Captive portal detection

Description of how Firefox detects captive portal networks.

Mozilla Crash Reporter TB

The Mozilla Crash Reporter dialog box appears when Thunderbird crashes and lets you send a crash report. Learn how it works and how to access your crash reports.

Fix "The bookmarks and history system will not be functional" error message

This article explains what to do if you see the error message “The bookmarks and history system will not be functional” when you start Firefox.

Diagnose Firefox issues using Troubleshoot Mode

Use Firefox Troubleshoot Mode (known as Safe Mode in previous versions of Firefox) to diagnose and fix problems. Learn more.

What do the security warning codes mean?

Firefox shows you a warning page about a potential security risk when you try to connect to a secure site but your connection is not secure.

Firefox uses too much memory or CPU resources - How to fix

Learn how to reduce Firefox's system resource usage for a faster, more stable browsing experience.

Troubleshoot and diagnose Firefox problems

This article includes a series of steps you can follow, that will help you fix most problems with Firefox.

How to troubleshoot security error codes on secure websites

Learn about error code SEC_ERROR_UNKNOWN_ISSUER MOZILLA_PKIX_ERROR_MITM_DETECTED and ERROR_SELF_SIGNED_CERT on secure sites and how to troubleshoot.

Resist Fingerprinting

Firefox's Advanced Preferences to resist fingerprinting can help prevent websites from uniquely identifying your device but can cause problems. Learn more.

Task Manager - see whether tabs or extensions are slowing down Firefox

Firefox has a feature that lets you see how much memory and CPU resources are being used by tabs, extensions and other processes. Learn more.

Firefox can't load websites but other browsers can

We'll explain errors such as "Server not found" or "Unable to connect" and how to fix problems where Firefox can't access websites but other browsers can.

Warning Unresponsive script - What it means and how to fix it

This article describes the reasons for this error message and covers possible solutions and workarounds.

HTTPS-First upgrades to secure connections

From version 136, Firefox implements HTTPS-First and always attempts to connect securely. If a secure connection fails it falls back to an insecure connection.

Rebuilding the Global Database

Rebuilding the Global Database will initiate a re-indexing of emails and address book cards (newsgroup messages are not indexed).

Firefox hangs or is not responding - How to fix

When Firefox hangs, it stops responding to clicks or keystrokes and seems to be frozen. This article has possible solutions, depending on when this happens.

DNS over HTTPS (DoH) FAQs

We have outlined a list of FAQs here that you may find helpful while getting up to speed with all that DNS over HTTPS (DoH) has to offer.

Troubleshoot Mode Thunderbird

How to use Thunderbird's Troubleshoot Mode to find and fix problems, especially if they are caused by add-ons.

How do I report a broken site in Firefox desktop?

How to report a broken website and what information Mozilla receives when you report breakage caused by Enhanced Tracking Protection.

Use the Troubleshooting Information page to help fix Firefox issues

The Troubleshooting Information page has tools and details about your Firefox installation, that can help you solve problems with Firefox.

How to let Firefox use the internet on your Windows computer

A "Server not found" error can be caused by Windows Firewall blocking Firefox from accessing the Internet. Learn how to configure it to allow Firefox access.

How to reset the default user agent on Firefox

Learn how to reset your Firefox "user agent" so that websites can detect your correct browser and version and work the way they should.

How can I tell if Mozilla VPN is connected?

Mozilla VPN displays visual indicators of its current status, making it easy to know whether your online activity is protected or not.

Websites look wrong or appear differently than they should

This article explains how to fix problems with websites that display incorrectly in Firefox or don't work the way they should.

Firefox DNS over HTTPS

DoH improves privacy by hiding domain name lookups from anyone lurking on public Wi-Fi, your ISP or others on your local network. Learn more.

Disable Firefox Sync

Learn how to turn off Firefox Sync if you’d like to use your Mozilla account without syncing data such as, bookmarks, history and tabs.

Certificate contains the same serial number as another certificate

Why Firefox may show you a Secure Connection Failed error when you try to access the configuration page for your router and what to do.

Where to go for developer support

Have questions about web development or Firefox Developer Edition? MDN Web Docs has many resources you'll need to build a better web. Learn more.

Firefox blocked by macOS 15 Sequoia firewall - What you need to know

Some macOS 15 users may have Firefox blocked by the firewall. Learn about the issue and how to help Mozilla investigate.

Connecting to unsecured networks while using Mozilla VPN

Mozilla VPN will alert you if you're connecting to an unsecured network.

Unload inactive tabs to save system memory in Firefox

Learn how to free up system memory by unloading inactive tabs in Firefox.

How to enable Always-on VPN on Android

Enable “Always-on VPN” on Android to encrypt your connection automatically every time you browse the Internet on your phone.

Mozilla VPN won’t connect or keeps disconnecting - How to fix

What to do if Mozilla VPN disconnects or turns off after being activated, or if websites fail to load even though Mozilla VPN is connected.

How to fix the Update Failed error message when updating Firefox

This article presents solutions and workarounds to a failed Firefox update so that you can get it updated and working correctly again.

Server Not Found - Troubleshoot connection problems

If you can't connect to a website, you may see the Server Not Found error message. Learn how to troubleshoot and fix this error.

Quick fixes if your Firefox slows down

The article covers the steps you can take to make Firefox run faster.

"Firefox is already running but is not responding" error - How to fix

How to start Firefox if you see an error message, "Firefox is already running, but is not responding" or, on macOS, "A copy of Firefox is already open".

Certificate warnings in Firefox for iOS

How to bypass certificate warnings in Firefox for iOS

Firefox does not work - Common fixes to get you back up and running

Find solutions to Firefox issues like crashing, not starting, slowness and websites not loading.

Drag and Drop errors with Firefox Launcher Process enabled and Windows UAC disabled - How to Fix

How to fix Firefox drag-and-drop errors when running Windows as Administrator with UAC disabled and the Launcher Process enabled.

Joining public WiFi while on Mozilla VPN

How to get online if you have trouble connecting to the Internet on public WiFi while on Mozilla VPN.

Compacting folders - Potential complications

Help the user understand some of the things that may go wrong when compacting.

Retrieve a Pocket crash log on Android

Troubleshooting Pocket for Android? Enable Developer Options & capture bug reports.

Firefox takes a long time to start up

This article describes some causes of a long start up time and steps you can take to improve it.

Thunderbird makes unrequested connections

Thunderbird sometimes automatically connects to the Internet. These connections are for routine maintenance, update and security purposes.

Some videos won't play in background

Some popular video websites have changed their settings on the server side so that videos will no longer play in the background.

64-bit Firefox crashes with Citrix's XenApp VM

The API hooks from Citrix XenApp 7.13 VDA do not work with 64-bit Firefox. How to find a workaround.

How to obtain the Mozilla VPN Logs

When troubleshooting issues with Mozilla VPN, our engineers can ask for logs. These don't contain any personal information, but give hints about the issue.

What is the Mozilla VPN kill switch?

This page explains a what a kill switch is

Report Site Issues on Firefox for iOS

Learn how to report website issues or problems in Firefox for iOS.

Solutions for "Can not create temp folder archive" error when installing Firefox (Windows 10)

Stuck with the "Can not create temp folder archive" error on 7-Zip when installing Firefox for Windows 10? This article will help you to solve the issue.

Fritz!box isn't working when Mozilla VPN is on

Fritz!box is blocked when Mozilla VPN is on.

Identify problems caused by third-party modules in Firefox for Windows

Identify problems caused by third-party modules in Firefox for Windows.

Difficulties opening or using a website in Firefox Focus for Android 100

A potential temporary solution if a web page is not opening correctly in Firefox Focus for Android 100.

How can I help submit and resolve bugs in the MDN content repo?

Learn how to help submit and resolve bugs in the MDN content repo.

Why do I see a white screen after updating Mozilla VPN to version 2.8.3?

Some Windows users who have upgraded to Mozilla VPN version 2.8.3 are running into a white screen and are unable to access most of the features when launching the VPN. In this article, you'll find a workaround for this issue.

How to find the Pocket app crash logs on iOS

To troubleshoot iOS app crashes in Pocket, find and email crash logs to Pocket Support.

RCS messaging and Mozilla VPN

RCS (Rich Communication Services) messaging on iOS 18 does not work when using Mozilla VPN. Learn how to fix or work around this issue.

Firefox uses too many CPU resources - How to fix

At times, Firefox may require significant CPU resources to display content. We'll show you options for fixing this if this if it is happening too often.

Websites say that Firefox is outdated or incompatible even though it's the latest version

Some websites that need a specific browser or version make a mistake detecting your installed version of Firefox. We'll explain how to fix or work around this.

Firefox says it's just updated every time it starts - how to fix

If you see "Firefox Updated" or "You've been updated to the latest version of Firefox" every time you start Firefox, we'll show you how to fix it.

How can I help by submitting mobile performance data?

The Telemetry and Crash Reporter features allow Mozilla's engineers to improve Firefox by sending back performance and usage information.

Memory constraints, Firefox performance and Firewalls

Firefox's performance can be adversely effected by memory constraint issues. Learn how to identify memory constraint problems and solutions.

Rebuilding Global Database

Rate your Firefox experience, surveys and out of date messages (Heartbeat)

You may be selected to rate Firefox or complete a survey. You may also see a notice to update Firefox. Learn more about these messages.

How to resolve weak crypto error messages on Firefox

When connecting to a website that uses an invalid TLS certificate or weak encryption, Firefox will display an error page saying "Your connection is not secure".

Crash reports in Firefox for Android

Learn how Firefox for Android handles crash reports, what data is collected, how it’s used, and how to manage the setting to automatically send crash reports.

Profiling Thunderbird Performance

How to create a Thunderbird performance profile to find causes of high CPU load or slowness.

Firefox for iOS won't load web pages

If a website isn’t loading even after you’ve checked the Internet connection, here are a few things you can try.

How to fix an error when trying to open Telegram or Slack on Firefox

How to fix the NS_ERROR_DOM_ABORT_ERR while trying to open web.telegram.org or app.slack.com on Firefox 87 or above.

Troubleshooting speed issues on Mozilla VPN

This article gives you a few troubleshooting tips to fix speed issues on Mozilla VPN.

The language of the websites I visit changes when I turn Mozilla VPN on - What to do

This article explains what to do if the language of the websites you visit changes when you use Mozilla VPN

Difficulties opening or using a website in Firefox 100

A potential temporary solution if a web page is not opening correctly in Firefox 100.

How do I contact MDN Plus support?

Learn how to contact support if you can’t find the answer to your question in the MDN Plus support knowledge base.

Where can I get help with the MDN API and documentation?

Learn the steps to report an issue with MDN documentation or the MDN website.

Firefox hangs when copying with Suggested Actions enabled on Windows 11 - What to do

Firefox can freeze or stop responding when copying, with the Suggested Actions feature enabled on Windows 11. Learn how to fix the problem.

How do I report an issue with Firefox Relay?

If you're having problems with any of your aliases, forwarded emails or the Relay extension, please submit a report so our team can work to fix it.

X-Mouse Button Control in Firefox 110

Learn about an issue with X-Mouse Button Control in Firefox 110 for Windows.

Address screen sharing issues with Firefox on macOS 15 Sequoia

Learn how to resolve screen sharing issues in Firefox when using macOS 15 'Sequoia' due to changes in Apple's API.

Bug 1931078

Bug 1931078

Crash reports in Firefox for iOS

Learn how Firefox for iOS handles crash reports, what data is collected, how it’s used, and how to manage the setting to automatically send crash reports.

Troubleshoot Firefox crashes (closing or quitting unexpectedly)

Learn how to troubleshoot and fix Firefox crashes, and find out how to get more help if you're having problems solving the issue.

Firefox is stuck in Troubleshoot Mode

Learn why Firefox may start in Troubleshoot Mode (previously known as "Safe Mode") when you expect it to start normally and how to fix it.

Firefox repeatedly opens empty tabs or windows after you click on a link

Firefox may repeatedly open empty tabs or windows after you click on a link, forcing you to close Firefox. Learn how to fix this problem.

Fix video and audio problems on Firefox for Windows N editions

If Firefox has trouble playing video or music on a Windows computer, you can install the Media Feature Pack, if it's missing. Learn more.

Avoid crashes - Tips and tricks

Here are some easy things you can do to avoid crashes and keep Firefox running in tip-top shape.

Problems with email and how to find help

Find out the right direction to get help using webmail in Firefox and links to support pages for popular web-based email services.

Firefox keeps crashing at startup

If Firefox keeps crashing when you try to start it and other solutions don't work, a clean reinstall can often fix the problem.

Home page with pipe character opens extra tabs when Firefox starts

Set up Firefox to open multiple home pages replacing the pipe character (|) with %7C.

How to run Firefox when your profile is missing or inaccessible

How to fix the error ''Your Firefox profile cannot be loaded. It may be missing or inaccessible'' so that you can start and run Firefox.