Primarily mochitest-gl, but also others. You can get an idea of what uses it by seeing what is blocking updating our ancient version of Mesa.[1] We anticipate that this update should improve the reliability of our tests.

I don't think there are specific results that are useful to anyone but us, other than that Mesa is viable for these sorts of things. We don't have a centralized tracking list for Mesa issues.

Like usual, if you try running WebGL on Mesa and run into unfixed issues, file a bug and we'll fix it.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1220658