The market-share bleed is ... amplified or mitigated, depending on your viewpoint ... by the absolute dominance of handheld Android devices (defaulting and manditory intalls of Google Chrome).
Firefox has slid some on desktop, but not nearly as much as broad numbers suggest.
Put differently: desktop is increasingly irrelevant to the modern Web. Which itself is likely not a good thing.
_______________________________
Late edit: Safari's marketshare, overall and by desktop/mobile segment, is virtually identical with its device share. Party like it's 1998.
Which suggests Mozilla's attempt at a mobile OS was not entirely misguided. It's preloads, bundling, and tying all over again.
If Firefox weren't bleeding market share this wouldn't be a problem in the first place.