Well, after each desktop Firefox update I ask myself, "Am I better off now than I was with the last version?"
In the Phoenix, Firebird and Firefox pre-4 days, I'd quite often find myself answering with "Yes". Since Firefox 4, though, I'm not sure if I've answered with "Yes" even once. Sometimes I'm not any better or worse off, but most times I'm worse off.
For example, extensions would break very frequently for a long time after Firefox 4 was released, although they eventually managed to get that straightened out. Then there's been the progressive dumbing-down of the UI, to the sorry state of affairs after the recent Firefox 29 release containing the Australis disaster. Useful functionality has also been removed, such as the status bar and the ability to disable JavaScript through the preferences dialog. With each update, a lot of us users now have to install more and more extensions just to restore useful functionality that has been removed.
Worst of all, I don't think there's been any significant improvement in its RAM usage, its speed, and other factors like those. Yeah, I know about the are-we-fast-yet style benchmarks, but those don't translate well to the actual experience when using Firefox. Chrome, as much as I dislike its UI, still feels far more responsive and efficient than Firefox.
As for Mobile Firefox, yeah, I'm comparing it to other mobile browsers from Google and Opera, for instance. I find it slower, I've had it crash more, and I don't think it really offers any significant benefits. If it's no better than its competitors, and worse in some ways, it inherently can't offer a good experience.
And it's nonsense to suggest that Persona was "widely used". It saw basically no adoption, compared to its competitors. The same seems to be happening with Firefox OS. Some people try it out, and there is some hype, but it's still a very, very marginal player in the big picture. It won't have any impact on "walled gardens" when almost nobody actually uses it. And in many ways it forces developers into a "walled garden" worse than that of its competitors, with JavaScript/HTML5/CSS basically being the only option for developing applications.
From a technical standpoint, the Asm.js versus Native Client debate is over. Native Client is a much more general, technologically-superior approach. Asm.js is basically just a human-unfriendly subset of JavaScript, without the benefits that a more general approach offers.
I think the Rust crew could have pulled off Rust 1.0 by the end of the year had they stabilized the language and standard library a few months back. But that didn't happen, and we're still seeing relatively significant breaking changes happening to this day. It just doesn't leave them much time all to freeze the language and libraries, and to then give it the significant amount of testing and bug fixing required of a respectable 1.0 release, before the end of 2014.