I am getting the youtube video. Something's off on your PC.
I have Firefox 31 and Flash Player 11. It worked fine all these years. Something tells me I'm not the only one having this problem
Judging from these:
http://forums.mozillazine.org/viewtopic.php?f=7&t=945195
& this: http://kb.mozillazine.org/Flash#Flash_player_not_working_on_certain_websites
I would try checking other websites/forums to see if the YouTube vids work there; and other flash content. If not, prolly deleting the YouTube cookies will work. If Flash doesnt work in websites all-around its probably an ad blocker or antivirus blocking it?
I can confirm that it does work on the other website im using
Why Firefox 31 ?
It's actually the latest stable version of K-Meleon. Same engine as Firefox, but works much faster and has a native Win UI.