The complaints about changing things, the whole CEO debacle, all of that is a drop in the bucket for people leaving.
You have Edge that ships with Windows, Chrome that ships with Android and both have major mega corporations pumping money into them and advertising for them. You might think it doesn't matter but for many people they will use whatever works and is the most simple.
If you really want to change Firefox's luck, break up Google, Microsoft, Amazon, Facebook and Apple, all five have a stranglehold on the internet that should not be allowed.
Sure, I think it's obvious that Google pushing Chrome so hard for years on their own platforms and elsewhere are primarily what gained them the 'majority' userbase. Add to that the knock-on effect of due to its increased market share the less devs tested as much with other browsers (leading to negative experiences with other browsers, sometimes intentionally by Google themselves), their tie-ins with Google accounts, extensions only compatible with Chromium, etc.
Similarly of course whatever MS includes by default will enjoy a chunk of the market share, too.
However an issue has been FF has also contributed to this themselves, for that percentage of their audience who make up power-users, previous fans, generally those who aren't just using a browser stock (for whom it would barely matter which browser they used).
Some things they deemed necessary like the Quantum addon change were nevertheless handled in a way that burned fans. If anyone was paying attention at the time they didn't implement the APIs necessary for various addons and features to migrate to the new system, despite developers requesting them via tickets, detailing them, blogging about them.
It was personally like experiencing a repeat of Opera's "under the hood" change in engines where they stripped out much of what its userbase had used and recommended the browser for (ironically it would be Opera's co-founder that left years prior that would deliver such desired features with the Vivaldi browser, also using the Chromium engine).
That itself caused that chunk of the userbase who had no alternative to a) stick with an older version (impractical long-term) or, including myself, ending up switching to Chromium (via Vivaldi). Even to this day FF lacks MHTML support that I use all the time and which it had for the longest time prior, including from an official Mozilla addon. It's UnMHT addon was
the best webpage saving addon bar none—even Chromium's native MHTML saving misses things UnMHT saves in 1:1 comparisons (apart from the superb auto filename character replacement feature, the lack of which still wastes time on Chromium).
There are other things Mozilla have done since that will have attracted some users (DNS over HTTPS, account containers) and repelled others (undesired changes, public messaging, whathaveyou). It's still important to have engine diversity of course though I couldn't return to it for regular use due to the useful things missing it once had.