I've been working at Mozilla for many years, from peak to decline. I can tell you exactly what's wrong with it. Nobody will tell you in their right mind though, and I'm tired to not communicating this, so here goes:
Mozilla has quickly been identified by a few as a way to make a quick buck. You see, it does not have share holders, no shares to give out, so execs gets really high salary in exchange with a 40% bonus every quarter that is almost guaranteed (everyone gets it, but 40% of 100k is +40k/y. 40% of 500k is 200k. that's 700k/year). Easy, when you get 400mi+USD and don't need much money to operate.
The problem is that they don't give a rats ass if Mozilla is successful, their metric is not financial (because its +- been assured to come every month through the single revenue stream: search deal), and it's not market share (because they have nobody to answer to except the employees and they tell us, I quote "market share does not matter much stop looking at it") (Fucking really Chris? REALLY?).
They also set their salaries, by the way. So basically they do random things they think are cool, with little to no data or idea of what matter or does not matter. Do you know half of them use Chrome as their main browser? How is that not telling?
We keep getting ridiculous thing after ridiculous thing. A lot of people opposed FirefoxOS vs getting back to the roots and attempting to do something about the web. FirefoxOS sounds like a nice concept, but everyone with a bit of a brain knew we had ZERO technical AND market chance.
Then, when it sounded like we're ok killing that and doing things well again BOOM IoT. Same mistake only even worse!
Oh as for when Brendan Eich got fired, yes, it was also ridiculous. But Psst. Mr Eich got the CEO position and a lot of execs were unhappy about that. He wanted to make Firefox the focus and make it a kick-ass browser. He started by changing everything we were doing. Sounded great! BOOM FIRED.
Mr Eich is now making the Brave web browser (based on Webkit by the way) which is arguably one of the most promising new browsers right now. Go figure.
Did I tell you about the story of our marketing and legal teams that did not want Firefox to ship with tracking protection? It took a long fight to get it in .. private browsing only. What about copying the stuff Chrome does well instead of copying Chrome UI? Like, you know, Sandbox, profiles, etc. Oooh nope not a priority. Adding ads (directory tiles) in the user's face THATS the priority (which got eventually killed because that did not even bring any money back, we only lost users to it).
TLDR most execs are corrupted pos, answer only to themselves, thus it's nearly non-savable and it will eventually die or be forked again as in the good old days (Rust+Servo anyone? These are THE good Mozilla projects around right now.)
a long time employee and contributor