Matt said

Overnight (I changed nothing, but microsoft rebooted my computer and I don't see any new files installed or updates) Firefox will not work. I can see it start up in task manager (two processes) but within 1 second both are gone. I've tried uninstalling and reinstalling firefox. Same problem. This is Windows 10 v 1909. Firefox will not start in safe mode. I cannot find any crash files (in the past month).

Download the relevent 32 or 64 bit release of the previous version from the links underneath, select as per your language

32 bit at https://ftp.mozilla.org/pub/firefox/releases/70.0.1/win32/ 64 bit at https://ftp.mozilla.org/pub/firefox/releases/70.0.1/win64/

Should work fine, once installed go options disable automatic firefox updates for time being.

Hope it helps

By the way do you have a AMD ATI graphics chip/card ?

Firefox starts now - thanks. Must be a bug in a new release. I have NVIDIA. Funny thing is, firefox works on my other computer with nearly identical installation.

So, the chosen solution is only a partial fix. Why? Because it only works 1 time. I can get firefox to start, but after I close it and try to reopen it, it won't start. I have to run the installation program again, then I can start it once.

This ended up being a Comodo problem. I uninstalled it a while ago, but the file remained and after I deleted it, it solved the problem:

C:\Windows\system32\iseGuard64.dll