Firefox only runs in Windows 8 compatability mode after windows 10 upgrade to version 1809

There have been cases in which a Windows update may break Firefox. Usually, this can solved by performing a clean reinstall of Firefox. This will fix any missing or damaged Firefox files, while preserving your data.

Hope this helps.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Manually back up and restore Firefox profile data.

Thanks for the reply Wesley, however I did a reinstall before going down the compatibility route.

Have you tried opening Firefox in Safe Mode?

Maybe some DLL were updated to versions that aren't compatible with Firefox or other software have loaded incompatible DDLs.

Anything in the Windows Event log?

• https://docs.microsoft.com/en-us/sysinternals/downloads/process-explorer
• https://docs.microsoft.com/en-us/sysinternals/downloads/procmon