Support for system libraries first appeared on mobile platforms because it was essential. At the time (ca 2011/12) the likes of YouTube and Vimeo were only delivering MP4 to mobile clients.
The purists seemed to think that somehow if Firefox held the line on (likely but not guaranteed) unencumbered formats/codecs that every content producer would see the light and go with those objectively worse codecs with no hardware acceleration for video content.
This completely ignored the fact that Flash was common on desktops and supported encumbered codecs just fine. It was in fact the primary delivery vehicle for them. The NPAPI content hole let the purists be intransigent while actually using content on the web. They were also ignoring the design of the HTML5 video tag that encouraged inclusion of multiple source formats and some sort of fallback including object embeds.
As for Microsoft they were still pushing the various Windows Media codecs as the One True Way in the Windows XP and Vista eras. They had to support MP4 with Windows 7 because every smartphone and most stand alone cameras were capturing video in MPEG-4 codecs if not file formats.
[0] https://groups.google.com/g/mozilla.dev.platform/c/-xTei5rYT...
https://support.mozilla.org/en-US/kb/fix-video-audio-problem...
All this stuff started in the days of Windows XP, which shipped with such a meagre selection of codecs you couldn't even play DVDs.
And the codecs you could download? Either they cost money (i.e. most users wouldn't have them), or they were dodging license costs by some questionable means, or they had bigger aspirations than just providing a mere codec (flash/realplayer/quicktime)
VLC arose out of that and became popular because, by ignoring the system codecs, it could actually play the damn file.