I have no particular love for Mozilla (no more than any other corporation), but your points all seem facile.

1) Regardless of whether anyone thinks Firefox's UI changes have been for the better, it's not possible to alienate Firefox users via UI changes because Firefox still offers the most customizable UI of any browser you've heard of. What are users going to say? "Damn you Mozilla, you made your browser look just like Chrome! I hate that so much, I'm switching to Chrome!"

2) Many of the remaining Firefox users point out that they have switched back to Firefox because Chrome has become bloated and slow. (Personally I think that everyone making this argument, on all sides, merely fails to appreciate what sort of benefit it brings to a browser to have a totally fresh user profile.) In overall benchmarks of memory usage and browser engine/Javascript engine speed, neither Chrome nor Firefox is significantly better by any significant margin.

3) Firefox for Android has between 50 and 100 million downloads on Google Play, and has a higher user rating than Chrome for Android (4.4 to 4.2).

4) Mozilla has announced earlier this month that they'll be shipping a Firefox for iOS, but given the crippling of third-party browsers on iOS I doubt Firefox will be any less hobbled than Chrome for iOS, and will certainly be worse than Safari.

5) I have no sales stats on Firefox OS, but given that they're still persisting in setting up new carrier partnerships I'd say they're better off than at least the Ubuntu phone. I'll probably never need a Firefox OS phone, but honestly if it weren't for the audacity of Mozilla trying to penetrate the OS market (and hence trying to end their reliance on the willing participation of third-party platforms to host their browser (which isn't so "willing" these days with the advent of locked-down platforms like iOS and WinRT (is that still even a thing?))), then I'd have already written off Firefox as dead in the water.

6) Thunderbird was never a moneymaker nor key to Mozilla's strategy, especially after the meteoric rise of web-based email clients.

7) Rust is the most interesting systems language to emerge in years, and its influence will be felt on every future systems programming language to come (though I am certainly biased here).

8) Servo is Mozilla's other project, aside from Firefox OS, that is so unbelievably audacious that I can't help but cheer them on. I have spoken with its developers and they're all astounded with the performance they're seeing, though they're holding off on releasing concrete numbers until the feature set is comparable with more complete browsers. Having seen the Servo devs in action, I can assure you they are on top of their game.

All this said, your original point was that you don't think that Mozilla has a top-notch team of C++ developers. And here's the thing: all of this is irrelevant to whether or not Mozilla's C++ developers are top-notch. I bet John Carmack's team at Id software was as top-notch a team of C++ developers as will ever be assembled, and yet Rage was still a commercial failure. To know whether or not the team is good, you have to look at their code and you have to look at their process.