Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

It is possible there is a problem with files in the profile folder that store the extensions registry.

Delete the extensions.* files (e.g. extensions.json, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder with Firefox closed to reset the extensions registry.

• https://support.mozilla.org/en-US/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• https://support.mozilla.org/en-US/kb/unable-install-add-ons-extensions-or-themes
• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled or not compatible extensions in ""3-bar" menu button or Tools -> Add-ons -> Extensions" then check if there is a compatibility update available.

• click the cogwheel button (left of the Search bar) on the about:addons page to "Check for Updates"

If this didn't help then delete add-ons.json and possibly prefs.js in case there is a problem with preferences.

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

Thanks for your answer. None of those worked, but it turned out that I had made another change: I was using the extensions at https://github.com/Aris-t2/CustomCSSforFx/tree/master/classic/css to improve the tab coloring (I was having trouble quickly seeing which tab was active). When I turned that off, everything got better.

I am glad to hear that your problem has been resolved. If you haven't already, *Please select the answer that solves the problem* YOURS. This will help other users with similar problems find the solution more easily. Thank you for contacting Mozilla Support.