Not sure whether or not this has anything to do with it–but could Netscape problems be due to the fact that it’s no longer supported ?
It should still work just the same, it just isn’t receiving updates anymore. 7.2 is pretty ancient though.
I had the exact same problem this week, with Netscape 7.2. Apparently, it has something to do with that older version of Netscape not supporting the latest version of Flash. I poked around the net and came to the conclusion the only fix was to change to another browser. I upgraded to Netscape 9.0 and Youtube is working fine now. (I know Netscape is no longer supported, but I’ve used it for so long and I want to keep on just a while longer…)
Hope this helps.
Grr!!:mad:
Nothing I try is working! Not for Firefox, anyway.
Youtube plays fine for IE. But like I said, I don’t want to switch search engines for one lousy site! WTF?
It looks like they made a recent change that affected older Mozilla-based browsers. Youtube is no longer working for me, but it still worked a week ago. I’m now getting the same message you quoted in your OP. I use Firebird 0.7, and I can live without Youtube.
Do you have any add-ons that are related? For instance there are some extensions that allow you to download flash videos, maybe there’s an issue there somehow.
I’ve added nothing! Nobody else has access to this computer. Nothing has changed. One day everything was fine, the next not.
I have the same version of Firefox with the same version of flash and it works, meanwhile you can play flash videos from other sites…does…not…compute… 
Check out the last part of this:
http://www.techsupportforum.com/alternative-computing/mozilla-firefox-browsers/171884-quicktime-youtube-problem.html
You got it! W…T…F?!?
And before anyone takes my crabbyness the wrong way, I appreciate everyones posts here, and mean no snarkyness except to the situation itself.
I’m having no other problems with no other sites in any way whatsoever. Why is this single one screwing with me?
Here’s another option though probably a long shot: go to Tools>Options and select the Network tab, there’s a box there to clear the cache, it’s worth a try.
I’ve noticed the same thing; I suspect that 3acresandatruck is right that YouTube changed some of its version-detection code and it doesn’t work right for Mozilla.
Here’s an ugly JavaScript hack that works for me. Follow the YouTube link to a page that gives the version error message (a page with a URL like http://www.youtube.com/watch?v=JzqumbhfxRo), then paste
javascript:u=document.URL;r=/watch\?v=([^&]*)/;if(m=u.match(r)){window.location.href="http://youtube.com/v/"+m[1];}void 0
into the location bar. This should give a full-window version of the video, until YouTube changes its back end again. (If this works, you can save this code as a bookmark so that you don’t have to cut-and-paste it for every video.)
Sonovabitch! It worked!! 
Thanks! 
Of course, now YouTube seems to be working in Mozilla again…