>>mconle+(OP)
TL;DR: Windows Defender had a bug that made certain system calls expensive on CPU cycles when Defender's Real-time Protection feature is enabled. After discovery, Mozilla reported this issue to Microsoft. Microsoft is releasing a patch that should result in lower CPU usage when using Firefox on sites like YouTube (a ~75% CPU usage reduction was noted when browsing YouTube in Firefox with the fixed version of Defender).
It seems like the HN submission form truncated the # from the end of the URL I linked to, which linked to the relevant comment. I'll try that here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1441918#c82
and
https://bugzilla.mozilla.org/show_bug.cgi?id=1441918#c91