[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode] {web link} A small dialog should appear. Click '''Start In Safe Mode''' (not Refresh). Is the problem still there?

Try to rename/remove xulstore.json and if present delete localstore.rdf in the Firefox profile folder with Firefox closed. You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:preferences</b> page. *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox ---- 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/ 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 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 *http://kb.mozillazine.org/Uninstalling_Firefox 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 then you can create a new profile to test if your current profile is causing the problem. <hr> *https://support.mozilla.org/en-US/kb/Profiles *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

Thank you both. Safe mode fixed the problem, so I am now working through the troubleshooting tips there. If I don't get anywhere there I will try the second approach.