It's really nice. Better than any other browser password manager. I've resisted using the browser's until now (I have 1Password) but the sync feature is slick for iOS.
Two things prevent me from switching away from 1Password: 2FA token support and shared credentials between multiple users (a shared family vault in 1Password). I could probably deal without the latter, but I definitely don't want to give up my auto-fillable 2FA codes.
The only thing holding me back is iOS support for changing the default browser. Here's to hoping the rumors about iOS 14 are true, and I'll be able to switch everywhere.
That shouldn't be a blocker for password managers. I use BitWarden, and it can auto-fill passwords in iOS Safari. So presumably there is nothing stopping Firefox from also sharing its password manager across whatever browser you use.