Yep. It's noteworthy that over 10% of Mozilla's users are on Linux. [1] And presumably that only includes users who submit the hardware report. That's a huge portion for any cross-platform application. It's genuinely rather surprising that they don't give it more attention simply because it's a large portion of their user base, and because Linux users are more likely to submit bug reports and patches.
I think there are good ideological reasons to support Linux, BSD, and other FOSS operating systems, not despite, but because of their unpopularity [0].
Edit: to clarify, I refer to operating systems that have a considerable amount of potential and existing development, not systems with like 2 users .
[1] https://news.ycombinator.com/item?id=21660399