FireFox fails to start -- including Safe Mode ?

[Yogi_Bear]

From my understanding since upgrading Thunderbird to 60.3.0; about three days ago. FireFox fails to start, including safe mode by means of command (DOS) or the -safe-mode command within the FireFox short-cut ?

[DanRaisch]

Close all applications and reboot the computer.
Delete any downloaded or partially downloaded Firefox installer files.
Clear the browser cache (of whatever browser you are using to download FF) and the Windows Temp directory.
Shutdown any anti-virus software you are running.
Download a new copy of the full installer from www.mozilla.org/en-US/firefox/all
Delete the directory C:\Program Files\Mozilla Firefox (or C:\Program Files (x86)\Mozilla Firefox
Run the installer.
Restart the anti-virus program.

[Yogi_Bear]

Close all applications and reboot the computer.
Delete any downloaded or partially downloaded Firefox installer files.
Clear the browser cache (of whatever browser you are using to download FF) and the Windows Temp directory.
Shutdown any anti-virus software you are running.
Download a new copy of the full installer from http://www.mozilla.org/en-US/firefox/all
Delete the directory C:\Program Files\Mozilla Firefox (or C:\Program Files (x86)\Mozilla Firefox
Run the installer.
Restart the anti-virus program.

Since I've never done a re-install; what about extensions and settings won't they be reset ? How can I create a backup, upon re-installing, I can simply overwrite those settings whether for both FireFox and all extensions ?

[LIMPET235]

All your "stuff" is located in the Firefox Profile & not with the Program files.
It will not be touched by this operation.
To be safe....back-up your Profile.
> http://kb.mozillazine.org/Profile_backup
An easy way is just to copy 'n' paste the Profile in the same place.
It will be re-named Profile (one) copy or similar.