1) If your market share is too low, sites don't bother testing in your browser. Since most sites are not written based on standards but rather by trying things until they work [1], this means that sites will start breaking in your browser and it will become harder and harder for anyone to actually use it. This is what happened with Opera and Edge.
2) If your market share is too low, you don't get much say in standards discussions.
It's not about having "big" market share. Having 90% market share or whatnot is a not really a goal for Firefox. But it does need "enough" market share that it can actually meaningfully achieve its mission [2]. Producing a browser that no one can actually use in practice and having no impact on the evolution of the web would both be failures in mission terms. 0.01% market share is "not enough". 20% is "enough". Where the line is in between is hard to tell.
[1] Hard to fault web developers for this; the standards are so complex that writing "to the standard", which requires understanding it, is hard. It doesn't help that recent web standards have had a tendency to be over-engineered, partly due to being created by a large company at the behest of other parts of that large company, so having a tendency to be designed to be usable by large companies but not necessarily worrying about anyone else.
1) If your market share is too low, sites don't bother testing in your browser. Since most sites are not written based on standards but rather by trying things until they work [1], this means that sites will start breaking in your browser and it will become harder and harder for anyone to actually use it. This is what happened with Opera and Edge.
2) If your market share is too low, you don't get much say in standards discussions.
It's not about having "big" market share. Having 90% market share or whatnot is a not really a goal for Firefox. But it does need "enough" market share that it can actually meaningfully achieve its mission [2]. Producing a browser that no one can actually use in practice and having no impact on the evolution of the web would both be failures in mission terms. 0.01% market share is "not enough". 20% is "enough". Where the line is in between is hard to tell.
[1] Hard to fault web developers for this; the standards are so complex that writing "to the standard", which requires understanding it, is hard. It doesn't help that recent web standards have had a tendency to be over-engineered, partly due to being created by a large company at the behest of other parts of that large company, so having a tendency to be designed to be usable by large companies but not necessarily worrying about anyone else.
[2] https://www.mozilla.org/en-US/mission/