This comment is perhaps the most accurate take on the fall of IE.
Firefox dethroned IE not because it was "MuH fReE oPeN sOuRcE sOfTwArE" or "MuH nEtScApE", it was because Firefox was superior to IE in performance and feature set. It was better than IE at being practical and enabling users to do things they needed or wanted to do.
Shortly after, Chrome rolled around and it was superior in performance (RAM hogginess aside) and feature set (for Joe Average, not necessarily power users) to Firefox and dethroned the dethroner.
Firefox will not be a dethroner again, because Firefox is not superior to Chrome. In fact, it's inferior to Chrome: It's been a third-rate Chrome ripoff for at least the last 10 years. None of the Chromium forks will dethrone Chrome either, because they are also third-rate Chrome ripoffs by their very nature.
It is conceivable that some browser superior to Chrome will eventually come out and dethrone it, but that's probably still a long ways away and I would argue the challenge of dethroning Chrome is several orders of magnitudes harder than dethroning IE ever was.
Which is probably why Google is pushing for so many web extensions - to make sure that they stay ahead of everyone else and you need billions just to catch up.
Unfortunately it seems Firefox is falling into the trap and is desparately trying to chase Chrome instead of focusing on significant user experience innovations to distinguish themselves from Chrome, especially ones that Google is unwilling to add to Chrome.
At least for me, Chrome was always faster than Firefox, though this always came at a substantial cost in RAM demands among other computer resources. This difference in speed has only become more and more apparent over the years as more and more websites have gotten bloated with reams of JavaShit and HTML5 nonsense.
I'm also of the impression that Mozilla and Google's respective marketing ultimately had a very minor impact on overall adoption. The key driver behind Firefox and subsequently Chrome's mainstream adoption was initial uptake by power users who then spread it to the commons by word of mouth. The latter is arguably still ongoing, seeing as OS-default browsers like Edge (let alone third-party browsers like Firefox) still can't even hope to compete with Chrome.
Originally, it was tabbed browsing, which Opera and Mozilla had ~5 years before IE. Yes, Microsoft sucked that much in those days.
With Chrome, it was an embedded PDF viewer and V8 performance.
So essentially, mostly people don't switch browsers unless there's a very good reason to do so.