Still – it’s a misleading title (border-line click bait). Anti-Firefox ad implies that they say something negative about Firefox, which clearly isn't the case.
"Still using Firefox?" Seems to me implies that it's old and antiquated and only used by people who don't know any better. Of course the opposite is true.
To build on your point: Yes, and this is precisely how Microsoft intended. It’s a disingenuous question. The OS knows you’re still using Firefox, because that’s what prompted the ad to begin with.