The difference is that when Firefox initially appeared, Microsoft was treating Internet Explorer as a "finished product" and letting it go completely stagnant.
Google and Apple aren't making that same mistake, at least not to that same level. The developers are happy with the tooling, and the fact that ECMAScript and HTML API's keep evolving faster than much of the ecosystem can keep up with. The casual public is happy about a fresh coat of paint, or putting tabs in a different place every few years.
There just isn't the same opportunity due to incumbent vulnerability today.
As I mentioned below, antitrust authorities are coming for Apple and Google. The opportunity will be there very, very soon; but you can't ride a wave without a surfboard, and at the moment it looks like Mozilla is more interested in selling product-placement stickers than building a great board.
Google and Apple aren't making that same mistake, at least not to that same level. The developers are happy with the tooling, and the fact that ECMAScript and HTML API's keep evolving faster than much of the ecosystem can keep up with. The casual public is happy about a fresh coat of paint, or putting tabs in a different place every few years.
There just isn't the same opportunity due to incumbent vulnerability today.