Market share was/is a means to an end.
Mozilla positioned Firefox in part as a reference for how web standards should behave (*), particularly for ones where Chrome/IE/Safari diverged with competing non-standard implementations.
But for that to work in the real world, not just at W3C debates, you need major websites to care if they function correctly on your browser. And for them to care, enough people have to be using your browser that these companies see a business case for spending money to add your browser to project plans and QA test matrixes as a compatibility target.
That decision is heavily informed by market share, whether global or as a percentage of the site’s own access records. In particular, double digits is a rough threshold for that, and that was pretty much Mozilla’s target. 50% would be wonderful but 10%+ let them assert standards in the ecosystem via the implicit threat of users leaving a site if Firefox didn’t work.
As a test professional at the time, one of the most discouraging things I saw after leaving Mozilla was Firefox dropping off all the test plans I knew about when they hit single digits. I’d poke at that decision where I had influence, and would basically get back a response that “Firefox is dead, just look at the numbers.”
(*) I’m pointedly ignoring some of the more aggressive introductions of things like device-interface APIs crucial to making a browser engine act like a phone OS, etc. Ultimately, someone has to build a working implementation before it’ll become a standard, anyway. There’s a race aspect for new ground and Mozilla was part of that.
But generally speaking, where there was an actual recognized standard, Firefox used it and not some homegrown alternative. So websites also had to develop to that standard to function correctly for a significant percentage of users.