I get a white screen with a thick blue bar at the top and nothing else. It just sits there unless I manually go into the Task Manager and kill it. This started with FF 90 beta. Previous versions updated fine.
PS: I always use the PortableApps built-in updater to update.
Please try manually downloading and temporarily installing it fresh to your Desktop directory or similar. See if it starts up. If it does, it means something is incompatible in your profile with the new release (extension, theme, custom settings, etc). If that's the case, you can start your existing install in safe mode (start FirefoxPortable.exe -safe-mode from the command line) and try disabling everything and then slowly enabling to see what breaks. If the clean install doesn't start, it's likely a GPU driver issue. Try updating your drivers if you can. If you can't update or are updated, start in safe mode and disable hardware acceleration in your settings.
Sometimes, the impossible can become possible, if you're awesome!
I'm was doing a fresh install with a new profile. No extensions, no nothing.
There's no old profile to speak of, so I'm not sure what in incompatibilities you mean?