Back in the day, web sites used Flash Player to show videos. Everyone's now moving off Flash and using HTML5 instead. Some sites detect that HTML5 doesn't work and revert to Flash, but maybe Facexxxx has dropped Flash entirely. I take it that your Firefox is an old version, probably about version 52 (the current version is 80), because Mozilla stopped support for Firefox on XP some years ago; if so, your Firefox may not do HTML5.
XP was good, but the world has moved on (not only Microsoft) and I'm afraid Specaircrew may be right: it's time to upgrade to Windows 10. If your system has 'loads of memory and CPU grunt' it can probably run Windows 10, and I think you'll find that it doesn't take too much getting used to.