Does it persist after you quit the browser (Ctrl+Shift+Q) and restart it?

You can try to reinstall Firefox in case something went wrong with updating. Do a clean reinstall of the current Firefox release and delete the Firefox program folder before installing a fresh copy of the current Firefox release. *download the Firefox installer and save the file to the desktop<br>*https://www.mozilla.org/en-US/firefox/all/#product-desktop-release If possible uninstall your current Firefox version to cleanup the Windows Registry and settings in security software. *do NOT remove "personal data" when you uninstall the current Firefox version, because this will remove all profile folders and you lose personal data like bookmarks and passwords including personal data in profiles created by other Firefox versions Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer. *(64-bit Firefox) "C:\Program Files\Mozilla Firefox\" *(32-bit Firefox) "C:\Program Files (x86)\Mozilla Firefox\" *it is important to delete the Firefox program folder to remove all the files and make sure there are no problems with files that were leftover after uninstalling Your personal data like bookmarks is stored in the Firefox profile folder, so you won't lose personal data when you reinstall or update Firefox, but make sure NOT to remove personal data when you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data. If you keep having problems, you can create a new profile as a quick test to see if your current profile is causing the problem. See "Creating a profile": *https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles If the new profile works, 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. *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile <hr> *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles Reinstall Firefox *https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems ----- If you use Sync, best is to disconnect Sync temporarily during troubleshooting. This could be a problem with the <b>places.sqlite</b> and <b>favicons.sqlite</b> files in the Firefox profile folder. *use the "Places Database" -> "Verify Integrity" button on the "Help -> More Troubleshooting Information" (about:support) page If errors are reported with "Verify Integrity", close and restart Firefox or reboot and retry. See also: *https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional *https://support.mozilla.org/en-US/kb/cant-add-change-or-save-bookmarks