We test on browsers we see more than 1% usage on by rule. This includes a swath of versions and platforms of IE, Chrome, Firefox, Edge as well as some foreign ones I know little about like UCBrowser.

Firefox Mobile does not meet that rule for us. I know personally a single person who uses it, and that's only because he's fiercely anti-Google.

There's just little sense testing against something no one is using, lest we want to test NetPositive [1].

[1] https://en.m.wikipedia.org/wiki/NetPositive