This is the one I'd be worried about. Thought it was annoying to not be able to use banking apps on a rooted Android? Think about how annoying it will be when you can't do much of anything, even on the Web, unless it's from a sealed, signed Apple/Google/Microsoft image-based OS...
I realize the way Firefox's user share is going, it might not matter or they might feel they don't have a choice but I really, really hope Mozilla doesn't even remotely consider implementing this.
Wouldn't it be great if you never had to deal with another captcha?
I run a custom build of Firefox, on a (somewhat, still-ish) niche Linux OS, with the kernel and bootloader signed by my own signing keys. What could I attest with, that will make some banking website perceive me as a trustworthy client?
The second this becomes widely available, it won't mean "bypass captchas" - it will mean "can't bank unless you use up-to-date Android or latest iOS".