Actually probably Mozilla’s largest user base

Why 'probably'? According to the stats I could find [1], Firefox has around 5% of Mac users, and around 9% of Windows users. Windows is Firefox's biggest platform, both in absolute terms and as a fraction of that platform's users.

On Linux, Firefox has a bigger percentage (>30%), but (desktop) Linux is much less popular than macOS so in absolute terms they have the fewest users there.

[1]: https://netmarketshare.com/

Mozilla’s Firefox Hardware Report has the latest desktop OS share across all Firefox channels:

  51% Windows 10
  27% Windows 7
   6% Windows 8
   6% macOS
  11% Other (Linux, BSDs, etc)

https://data.firefox.com/dashboard/hardware

I think the Firefox Nightly, Developer Edition, and Beta channels have a slightly higher share of macOS and Linux users, but not much. There are a lot of Windows users out there; you just don't see them at JS conferences. :)

The plan is to do user testing on mac and if it is validated roll it out.

Replay requires a ton of difficult platform-level investment to get right and we're still actively finding bugs in the real world.

After it is validated, it will be easier to add the appropriate platform level bindings in other OSs.

I'm really disappointed that Linux gets less attention from FOSS-supportive organizations like Mozilla. Desktop Linux adoption is a catch-22: people don't use it because it doesn't have as much software; however, it has less software because it's less popular.

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.

[1] https://news.ycombinator.com/item?id=21660399

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 .

[0]: https://news.ycombinator.com/item?id=21660819

The statistic that matters for this tool is the distribution of developers, not the overall user base.