I’ve never been unable to access a site on Firefox due to DRM. There is a prompt asking to run DRM-enabled media, but that’s it.

Edit: or is there something about Manifest v3 that will get Firefox blocked somehow? IDK how as I would think it would be easy to pretend to be compliant.

If you have the funds, donate to Mozilla. They’re not only the main developers of the only major competing browser engine, but also do a lot of other good work. You can hope for others, but with Firefox only having single-digit usage share it needs all the help it can get.