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.