People say that but for ages FF simply couldn't do what IE did. ActiveX was a thing and FF just didn't support it. Java applets broke all over, and CSS support was a minefield.
Most of the mainstream web was outright broken in MozSuite and FF. It wasn't clear at all that FF was "superior" in any way - in fact, the build system was a nightmare, you couldn't embed it anywhere, you had to fix proxy settings all the time (because it had its internal stuff, not using the Windows settings)... Yeah it had extensions, but a lot of people didn't even know what those were (and making them wasn't particularly easy - easier than dealing with MSVC++, but still not easy). Still, people were
invested in the success of FF; they tolerated the brokenness and pestered the hell out of website owners to fix their shit and evangelized the browser to friends and family. Because people
cared about FF and what it represented: an open browser to use the open web, built for people and geeks - not "consumers" or "enterprise customers".
Now people don't care, because Mozilla is seen as Just Another Silicon Valley Corp, with overpaid execs doing shady deals to push shit down our throat. At that point, "they are all the same" so might as well use the browser that works more often (Chrome). This can change, but it needs a positive shock.