As a quick hack on ubuntu you can revert 121.0.1 with sudo snap revert firefox,
but you need to create a new profile.
You also need to reinstall the RealUser XModule and re-add your licence…
Interestingly in ff 121.0.1 the vision module (shutter is installed) and the DesktopAutomation XModule - Visual Desktop Automation is not recognized. (Even with player license)
@admin is there a fix coming soon?
Thanks for your help.