looks like rendering adblockers extensions obsolete with manifest-v3 was not enough so now they try to implement DRM into the browser giving the ability to any website to refuse traffic to you if you don't run a complaint browser ( cough...firefox )
here is an article in hacker news since i'm sure they can explain this to you better than i.
and also some github docs
I think they're referring to the storage service at sync.com, not Firefox Sync.
Correct
How do you like it? Would you prefer it over dropbox, or not really?
I’ve been migrating to sync from Dropbox after hearing too many reports of Dropbox scanning user content for things they deem objectionable. I like the end to end encryption, but I have found the mobile experience on iOS to be lacking. It seems to have trouble integrating with Files and uploading files directly via the iOS share menu. Annoying but not a dealbreaker.