If the videos is using a HTML5 player and not using the Flash Player Plugin then that can help explain why you cannot view some videos.

WinXP does not come with media feature packs that Microsoft provides to Windows Vista and Later. The packs have codecs needed in order for web browser like Firefox and SeaMonkey to fully using HTML5 players for video and sound.

This is for Windows Vista and later users but explains what I mean about the packs. https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows

You can see the limit of what you support at https://www.youtube.com/html5/ as you likely have two or three red check marks

Chrome may seem to work but that is because they kind of cheat by coming with own codecs. Also Chrome, Chromium, and Opera dropped support of WinXP/Vista back in April 2016 so the last versions that worked are old and vulnerable on WinXP now.

With some work you may be able to support HTML5 players better in Firefox 52 ESR on WinXP. https://msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/

btw the browser we know as Firefox (third name) started as of Phoenix 0.1 in Sept 2002 so not quite 20 years yet.

Thanks, James, I did all the things you said to do. Downloaded Zipped Primetime put it in sub-folders. Went to FF about:config, did the things you told me about. It did not work because I had put semicolons in the Preference Names on 2 of the strings, I took those out and now, it works. Problem solved. It was difficult, but, I finally found how to do things in about:config. Thanks again.....GGlynnD

GGlynnD said

Problem solved.

Since making the changes do you find your browser more prone to freezing when a page has content that causes Adobe Primetime CDM to run? I have noticed more freeze issues.