Which FF version are you refering to and which device and OS?
I am on macOS Monterey v12.2.1 and FireFox v100.0b9 (64-bit) and it still works as it did in the past.

EDIT:
Could you please follow this guide from the discussion board? It used to be the Chrome only issue (on Windows), but maybe it is now FF too.
https://github.com/dvlden/ultrawideo/discussions/62