Hi, first restart Firefox. If that doesn't help, hold down the Shift key when you start Firefox to start in Safe Mode. More - Diagnose Firefox issues using Troubleshoot Mode.

If it works in Safe Mode, disable any theme or extensions you have - 3-bar menu > Add-ons > Themes/Extensions and restart Firefox; then if it's OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some extensions.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

Have restarted Firefox numerous times; the problem still existed.

Restarted Firefox in "safe Mode" numerous times; the problem still existed.

Are you using Restore Previous Session?

Did you try to open a new window?

You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data.

• delete sessionstore.jsonlz4 and sessionstore.js if present

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

We cannot know a user's technical skills, so if you find cor-el's suggestions are too daunting, here's a couple of easier things to try:

Open the 3-bar menu > Customize, and click on Restore Defaults in the bottom right corner.

If that doesn't help, I'm going to suggest you try Refresh, but since it can very occasionally cause issues of its own, to be on the safe side, although not essential and before you do it, I recommend that you backup your bookmarks: Open bookmarks > Show All Bookmarks > Import and Backup > Backup. Or, Export Bookmarks to HTML, and save them to somewhere convenient on your computer. Note that Backup will overwrite any existing bookmarks when you restore them. For extra safety, I'd suggest making both a Backup - which creates a JSON file, and an HTML file. You can instead backup all your data by copying your Profile, but this requires some technical expertise - Back up and restore information in Firefox profiles.

The Refresh feature (called "Reset" in older Firefox versions) can fix many issues by restoring Firefox to its factory default state while saving your bookmarks, history, passwords, cookies, and other essential information.

Note: When you use this feature, you will lose any extensions, toolbar customizations, and some preferences. See the Refresh Firefox - reset add-ons and settings article for more information.

To Refresh Firefox:

1. Open the Troubleshooting Information page using one of these methods:
   • Click the menu button , click help and select Troubleshooting Information. A new tab containing your troubleshooting information should open.
   • If you're unable to access the Help menu, type about:support in your address bar to bring up the Troubleshooting Information page.
2. At the top right corner of the page, you should see a button that says "Refresh Firefox" ("Reset Firefox" in older Firefox versions). Click on it.
3. Firefox will close. After the refresh process is completed, Firefox will show a window with the information that is imported.
4. Click Finish and Firefox will reopen.

Did this fix the problem? Please report back to us!

Thank you.