I've pressed down alt to open in safe mode

You need to press and hold down the Shift key when starting Firefox on Windows (Option key on Mac) to get into Troubleshoot Mode.

On Windows, your Microsoft account may have parental controls enabled which limits what browsers you can use, you could try to get this limitation removed or create a new account.

You may be able to get it working by renaming firefox.exe to something else but this will break DRM protected videos.

When trying to open it from my Mac Terminal, I get this response:

The application cannot be opened for an unexpected reason, error=Error Domain=RBSRequestErrorDomain Code=5 "Launched process exited during launch." UserInfo={NSLocalizedFailureReason=Launched process exited during launch.}