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.
Online fraud and theft is exploding right now and the average person is simply not capable of securing a laptop so the companies have decided to only allow secure access through a phone which can usually be trusted to be malware free.
As long as Windows users are allowed to remain as out of date on patches as they are, and depending on what the browser users as its attestation "source", I don't see how the browser and website can ever meaningfully establish the validity of the statement "the client is trusted to be malware free".
It's too hard for even someone who is highly knowledgeable to know if they have malware, let alone the average person.