Firefox hasn't been in a good place lately. Most of the changes since Firefox 4 haven't been to the users' benefit. It's like these changes have been more about imitating Chrome, regardless of whether or not this is good for Firefox's users. Yet Chrome is still superior where it really matters, such as performance and resource usage. If Firefox users are just going to get a Chrome-like experience these days, but not as good as that offered by actual Chrome, then they might as well just move to Chrome. And I think that's exactly what we've seen, and why Firefox' usage numbers are dropping.
We see the same with Firefox Mobile. It really isn't superior to the alternative browsers in any way. In many ways it's significantly inferior. There's really nothing to pull users to it.
Thunderbird was perhaps their second most useful product, after Firefox. Yet they've basically given up on it now. I know I've migrated to another email client, and many others have chosen to do the same, too, now that Thunderbird doesn't really improve over time. Even then, the changes we've seen lately have been more harmful than good. The UI is less usable now than it was in earlier release, for instance.
Firefox OS is another example. It really doesn't offer anything tangible over Android, iOS, or the many other mobile OSes that already exist and already widely available on many devices. In fact, it offers a very limited development environment compared to the alternatives, which surely doesn't help its case. The only reasons I've heard to use it are ideological, about Mozilla somehow being "more open" or something of that sort. That's just not enough to gain real traction, I'm afraid.
And Persona is yet another example. It just doesn't meet the needs of its potential users.
Rust is perhaps the only interesting thing I've seen coming out of Mozilla lately. But it has taken a long time to get to where it is, and I'm not sure if it still has the momentum to have the impact that it might have had were a stable version available a year or two ago. Go, and even C++11, can now provide a lot of its benefits today, if not much earlier.
I think there's been a lot of wheel-spinning at Mozilla lately, in terms of their offerings. Their successful products are being ignored or actively made worse, while their new offerings don't actually benefit a wide audience, or offer an insufficient amount of benefit.
I'm not going to pretend to know how to fix these problems, but focusing on product that users actually want, and giving these users the functionality they want, may be a good start.