Changing behavior based on user agent is necessarily intentional on the part of Microsoft.
That check lies somewhere along the line between "having the direct goal of breaking authentication flow (pure malice)" and "is a completely legitimate programming error (pure incompetence)."
I am not ready to assume pure incompetence (and here's where I might be wrong).
Ah I see, I thought the parent poster meant malice on the part of Mozilla, got confused by bouncing between comment threads. I could see malice, since it is Microsoft, but what's the "why" of it? I don't really see any motivation that M$ would have to block Mozilla, all it's going to do is piss off users. It's not like people are gonna get fed up and switch to Edge, they'll get fed up and switch to Chrome. If anything, M$ has a great incentive to improve Firefox adoption. The market that uses FF is the same market that is never going to choose Edge. FF and Edge both have a much better position if they can damage Chrome's market share.
