Is there an error code (usually in all caps, along the lines of SEC_ERROR_SOMETHING) on those pages? Please copy/paste that into a reply (here at https://support.mozilla.org/questions/1418021, not by email).

Spoiler alert: we'll be looking it up in How to troubleshoot security error codes on secure websites.

PR_CONNECT_RESET_ERROR

That is the error I get when trying to load your website

I use Kaspersky, and it is up to date. It updates automatically.

When I try to disable the "scan encrypted connections" feature, I get a warning. Why should I disable something that is protecting me, and ISN'T interfering with my access on Chrome?

"PR_CONNECT_RESET_ERROR" means that the web server, or your router, or some other device at a waypoint between Firefox and the site sent a RST response code which instructs Firefox to immediately stop trying to connect. But what/who would do that for every site?

Before testing what happens if you turn off the Kaspersky feature, can you visit non-secure addresses, or do those fail as well? For example:

http://www.example.com/

When I go to the link you provided, I get the following error:

The connection was reset

The connection to the server was reset while the page was loading.

   The site could be temporarily unavailable or too busy. Try again in a few moments.
   If you are unable to load any pages, check your computer’s network connection.
   If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the web.

Why is this NOT happening when I use Chrome?

And why did it start happening the instant I downloaded the most recent Firefox update? when I restarted Firefox after the update, every tab came up with an error.

Thank you for confirming that the same problem occurs with non-secured connections.

It sounds like something on your computer or network doesn't want the updated version of Firefox to be able to access the Internet. I don't know whether that might be Kaspersky, Windows Firewall, or something else. You could check those two programs and see whether they have a feature to prevent programs from connecting to the Internet and make sure Firefox isn't blocked there.

Or maybe your installation of Firefox is broken. You could try the following:

Clean Reinstall

This assumes you run Firefox downloaded from Mozilla rather than from the Windows store. This might cause data loss with the Windows store version.

This takes about 5 minutes. This doesn't clear your settings, but it removes folders that might contain Policy and AutoConfig files that can override standard operations. It is not necessary to uninstall Firefox, but you can if you like, saying No to any request about removing personal data.

(A) [Optional] Back up your Firefox data. See: Back up and restore information in Firefox profiles.

(B) Download a fresh installer for Firefox to a convenient location:

https://www.mozilla.org/firefox/all/#product-desktop-release

(C) Exit out of Firefox (if applicable).

If you use Microsoft Office, please change your default browser to Edge or Chrome temporarily before the next step.

(D) Using Windows File Explorer/My Computer (hold down the Windows key and press E to launch it), right-click > rename the program folder as follows (you might have one or both):

Open C:\Program Files and look for Mozilla Firefox

If you see it, right-click Mozilla Firefox, click Rename, and rename it to OldFirefox

Open C:\Program Files (x86) and look for Mozilla Firefox

If you see it, right-click Mozilla Firefox, click Rename, and rename it to OldFirefox

(E) Run the installer you downloaded in step (B). It should automatically connect to your existing settings.

Any improvement at your next startup?

For Step B above:

I wonder whether any of the fixes in the 115.0.2 update will help (it's sometimes hard to tell when failure to connect is due to a blocked process):

https://www.mozilla.org/firefox/115.0.2/releasenotes/

I've got the exact same problem as you, OP. I'm using Microsoft Edge to make this post.

First of all, thank you for continuing to help me! I appreciate it.

I downloaded release 115.0.2 and the problem didn't go away.

In case my copy is corrupted, I will try do a clean reinstall (thank you for the instructions) but I have too much work at the moment to stop and restart my computer multiple times (it takes forever to reboot). I will try and do it tonight. After using Chrome for two days, I hate it! But I need to finish this job first.

AHA! I'm glad I'm not the only one. I wonder how many others don't have a backup browser they can use to access support.

In case my copy is corrupted, I will try do a clean reinstall (thank you for the instructions) but I have too much work at the moment to stop and restart my computer multiple times (it takes forever to reboot). I will try and do it tonight. After using Chrome for two days, I hate it! But I need to finish this job first.

As far as I know, there's no need to reboot the computer. If your security software says you need to restart so it can get its hooks into Firefox after the change, you can defer that until you test whether the replacement worked.

First, I disabled encrypted connections scanning in Kaspersky, rebooted, and then opened Firefox. No change - still unable to access the internet.

I restored encrypted connection scanning, rebooted, and opened Firefox. No change.

I renamed the old Firefox in Programs, downloaded the new one, and opened Firefox. No change.

Now what?

I just disabled encrypted connection scanning but did NOT reboot. When I opened Firefox I now get a different errors:

Most pages clock and never load or error out: Amazon, Facebook, eBay, and the weather channel.

Some pages load, but if I press any button on the page, it clocks and never loads or errors out: eBay (yes, some pages load, others don't).

On some pages, I get this:

Oops! Something is wrong Error 500: Internal Server Error Server doesn’t respond or something else happened. Please, try to refresh this page.

When I enable encrypted connection scanning, it returns to the previous error:

Secure Connection Failed

An error occurred during a connection to www.facebook.com. PR_CONNECT_RESET_ERROR

Error code: PR_CONNECT_RESET_ERROR

   The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
   Please contact the website owners to inform them of this problem.

I just disabled encrypted connection scanning but did NOT reboot. When I opened Firefox I now get a different errors:

That would suggest Kaspersky is contributing to your problem. Visit about:third-party in the address bar and try blocking any third-party modules. Does it still happen in Troubleshoot Mode?

Update: I contacted Kaspersky and they had me uninstall/reinstall my virus protection which seems to have been the problem. From what I've gathered, it looks like this has been a problem (on their end) for years. >.>