As a test, disable your protection programs.

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
  *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 then you can create a new profile as a quick test to see if your current profile is causing the problem.

• 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

As a test, disable your protection programs.

Thanks, Yes it appears for some reason Trend Micro Application Control has started to block Firefox. I will check with our IT department and see why this changed. I found this digging into the Security Agent event logs with type "application control". I guess Trend Micro can have a permitted apps list, maybe the most recent Firefox update and a Trend Micro update are clashing suddenly. I don't seem to be able to disable Trend Micro to confirm, but since I see it is doing blocking I think I can safely assume that it is the reason.

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems.

Be sure to contact Trend Micro support.

It may have been an issue by Trend Micro. The Frefox 81.0.2 update today solved the issue, so I think Trend Micro themselves somehow were not allowing 81.0.1 to run. My IT department said they didn't change anything, it just started working today after the Firefox update installed.

Good to hear.