User agent strings aren't really seen by people so arguably there is no consumer confusion. And if you need the Mozilla user agent for compatibility it's reminiscent of https://en.wikipedia.org/wiki/Sega_v._Accolade

The mark in this usage has gone unprotected for so long, I doubt there would be any success with that.

Also, I doubt tricking servers would indicate creating consumer confusion with the trademark.

Yeah, but the case law resulting from the denied ruling could prove valuable for others who need to defend their use of trademarks in API naming. "API" here refers to the broad judicial term, where the User-Agent header falls under.

See Sega v. Accolade for a precedent of why that likely won't work.

Adversarial compatibility is pretty nice though, I don't think we should do away with it.