That's Safari. It has way more users, and website creators actually care about providing Safari support because they want it to work on the non-technical boss's iPhone.
people typically want their mobile sites to work on both major OSes
Or on Windows.
There's no significant Firefox presence on Android either. Sure there is an app, and there are 100+ million installs, corresponding to an install market share of >4%, but people almost never use Firefox. Usage wise, Firefox has a 0.5% market share.