Which is it, because the headline here says 'Beta' but I'm already on the Beta channel and see no update and my architecture is still 'Intel' in Activity Monitor.
Should I choose "Nightly" in the dropdown to get the ARM64 universal binary?
Are you on 84b1 or more? If so, try exiting and restarting it once manually. If you upgraded from a version running with Rosetta, the restart initiated by Firefox will keep it running with Rosetta. This is tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1677036
I had the same situation, seems the Beta auto-update doesn't switch to the new architecture. Just download the beta again and install it over you old install.
The performance difference is _really_ noticeable :)