The Firefox market share was eaten largely by the enormous and legally dubious marketing campaigns by Google and Microsoft. Hard to see how Mozilla could compete with constant forced nags and defaults in the most widely used websites and operating systems.
It was a big factor, but so were things like the way they treated their mobile browser for years and years, which is the platform 2/3 of browser traffic now originates.
According to statcounter's stats, Firefox never cracked 1% of monthly mobile traffic any month from when stats started in 2009. Even Opera and UC have more than double Firefox's average for the last year and they are just Chromium forks users are downloading off the stores.
For context, I recall that for years and years, Firefox was the highest ranked mobile browser. Mozilla invested a lot in mobile, Firefox devs had to rewrite the Android linker, invent new ways of starting binaries on Android, etc. just to make Firefox work (all of which were later used by Chrome for Android).
It still didn't make a dent in mobile browser shares.
Sure, Mozilla could have invested even more in Firefox mobile, but at some point, this would have come at the expense of Firefox desktop, which was the source of ~100% of the funding.
What Firefox was doing 4 months after Android 1.0 GA'd would indeed been unlikely to have made a dent in mobile share compared to what effort was going on once Android had a billion users. Why put all of that effort in before something is even used to just then let it rot for years anyways? In the end, they ended up spending the resources to refresh it in 2019 anyways - by which time billions had already decided Firefox was just a battery hog and slow on mobile.
It's a sad story because Firefox was so good on mobile when nobody had a chance to use it then it was crap when they did. On desktop Firefox is still the #1 non-bundled browser, things went so poorly on mobile they can't even come close to that today. In a parallel universe timings were inverted and Firefox may have even had more users on mobile than it does desktop today.
It was eaten by Google flexing their search and mobile monopolies, to advertise it on one of the most visited websites in the world, and coming pre-installed on over a billion devices. Distribution lock in leading to market share dominance is a tale as old as computing to the point that we now have decades old case law specifically on the issue of software monopolies being the cause of browser market share dominance.
Interestingly the most recent anti-trust case against Google, one proposed remedy was spinning off the Chrome browser into its own company, but that option was judged to be unrealistic, because how would a browser survive on it's own without distribution advantages and all its costs subsidized by other revenue drivers? A great question.
There's a lot of opinions and anecdata in that. Firefox was almost never as bad as it was marketed to be (by its competition), and Chrome was certainly never as good as it was marketed to be (by an evil ad company pretending to be a good, well adjusted internet benefit company).
Firefox used to be the first browser with decent developer tools thanks to the Firebug extension.
Then Chrome pushed heavily the development of its own development tools and crushed Firefox in the process. Chrome has now been the best browser for developers for the last +10 years.
The consequence is also that when the new generation of headless browsers were developed (Puppeteer) they were based on Chromium because it is so hackable and developer friendly.
This means that Firefox lost a big chunk of the developer community which constituted also a non trivial amount of their user base and advocates.
I don't know why people left Firefox, but I know why *I* did. And it was three or four years ago (after using it for 15 years) because I got annoyed at them for removing many features that I used over the years, and because I tried a Crommium based browser and it just had better performance and better ad-blocking. That's just one anecdote, but feel free to correlate it with other anecdotes to find some patterns and reach your own conclusion.
My father, who is very non-technical has never left Firefox and stuck with it for decades, even against Microsoft and Chrome's tactics to try to claim default browser and constantly install them into his face. My father particularly hates Chrome because he never understands how it keeps reinstalling itself despite his best efforts. His taskbar is often a mess of all three browsers because he can't figure out how to keep Edge and Chrome unpinned. My father sees Chrome installing itself and auto-pinning to his taskbar and Start Menu as the exact same IE-level adware/spyware shenanigans that led to him fleeing to Firefox in the first place.
I returned to Firefox again after years of IE8+ and Spartan Edge. I've never liked the "mouthfeel" of Chrome, have generally felt it to be bloated and slow and ad-heavy adware (though not as strongly as my father and I often do know how Chrome gets backdoor installed through shameful adware deals like with Adobe), and when Edge switched to being just another Chromium I still felt the same in my dislike of Chromium and I went back to Firefox. (Spartan Edge had so much better performance and battery usage than Chromium. It's death was not mourned by enough people.)
Feel free to correlate these two counter-anecdotes with more and see if you find some patterns to reach your own conclusion. That's the fun of anecdata and marketing, there are patterns on every side, you can interpret it how you want. "Popularity" isn't facts, pattern matching based on popularity of certain anecdotes can lead to incorrect conclusions. Especially when Marketing is involved. Marketing is about making popular things that aren't necessarily facts, especially when an advertiser is unscrupulous and no one is busy enforcing truth in advertising laws.
These anecdotes say the people still using Firefox don't like Chrome/Edge and that few cared when Edge switched from Spartan to Chromium. I don't think anyone disagrees with any of that. It's the anecdotes about the different reasons people actually stopped using Firefox for under debate, not the reasons a few have still stayed anyways.
I.e. IE (couldn't resist :)) can be said to have used the exact same shenanigans, as mentioned above, but there were other reasons droves of people still decided to install and use Firefox back then anyways. People no longer make the same decision to install and use Firefox, so if the shenanigans haven't changed... then what did? This is where the common refrains that Chrome managed to be a better browser (particular on mobile) for that decade or that Firefox managed to regress in certain ways come from. Sure, Chrome absolutely got its growth blasted forward by marketing and bundling, but people decided to stick with it and stop using Firefox for reasons unrelated to that. Sometimes niche reasons, sometimes general reasons, but the story was never something like "Chrome invented marketing and bundling, which resulted in Firefox losing its easily gained massive market share of the time".
No, Chrome was genuinely better than Firefox back then. Firefox didn't have multiprocess until many years later and Flash constantly crashed the browser.
