Firefox relies on the Microsoft "Media Foundation" for decoding MPEG media (like MP4 video) in the HTML5 player. Microsoft never released Media Foundation for XP, only Vista and newer.

I don't know whether there are any other plugins that could help, or whether you'll need to use an external player, for example, get the link to the video and feed it to VLC.

VLC: https://www.videolan.org/vlc/

You may be able to support HTML5 better with the work around mentioned in http://www.msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/

Alternatively you could install a light 32-bit Linux distro with a light manager like Xfce or such (avoid Gnome3 and KDE) in a dual boot with WinXP to be much more current and secure compared to the old EOL WinXP. Just make sure you install packages like FFmpeg so you can support HTML5 fully.

This looks like a headache and probably beyond my computer skill level to try and do. Is there definitely no alternative video player to replace Flash that works out of the box or with minimal easy changes with XP and FF 52.9.1?

Videos still play on youtube but that's it,nowhere else.

The Adobe Primetime module doesn't work. It shows in the add on's list but won't install. Complete waste of time after I figured out how to install it. My antivirus detected a virus in the Mozilla profiles folder which probably came from the Primetime zip file.

The easiest solution to the problem is to ditch Firefox and use the Mypal browser for XP which plays videos that are not on youtube and as well on youtube,plus the Mypal browser gets updated regularly unlike old Firefox browsers that work with XP.