If you design your thing for the average user, it won't work well for any user.
The only benefit Firefox now has over Chrome is a vague political notion of not using Google-made software, something that there isn't enough call for in the real world to drive significant marketshare. Their real differentiation was sacrificed on the altar of ease of development, and now what we're left with is an also-ran, in multiple meanings of that term.
Worse, an also-ran that's doing increasingly arational things, almost as if they're wildly groping about for the mythical marketshare.
A majority? I doubt it. I think it’s an extremely vocal minority that don’t care about the slowness of their browser (pre-e10s, pre-quantum), about things breaking (XUL extensions) or about UI features for the broad masses.
I’d bet money that FF’s 25% in Germany would be far closer to US numbers if they stayed where those vocal people want them.
All I'm saying is that based on my observations people's views on what Firefox should or should not be are different, and that this explains why people are still "angry" about the extensions breaking and such. I'm not having an argument with anyone.
If anything, using the MUA bundled with your browser was less power-userish than using a stand-alone client.
Honestly, Opera is one of the best pieces of software I've ever used. It had tons of features and customizability without being slow or feeling "bloated". I never quite understood why it failed to gain marketshare 10 years ago while Firefox did.