I work at MS, tried to use Firefox but couldn't because FF doesn't integrate with the Windows cert store. Crucially, this keeps Windows Hello (TPM auth) from working, which makes it useless for any internal websites. For a while I used a hand-compiled PKCS#12 plugin that bridged to the cert store, but that was extremely fragile and eventually I gave up.

I think this is probably a major blocker for many enterprise users, and wish Mozilla would have fixed it.

edit: it looks like they may have fixed this in the past couple years, though you might have to go poking around in about:config.