Hi,

Are you sure this is a firefox problem? Can you watch the same videos on IE? Because it's possible that your video "is no longer available" or exist. That means that your videohost have got some server problems or that your video has been deleted.

Kind regards, Joren

"Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
"Remove the Cookies" from sites that cause problems: Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Use Troubleshoot Mode in Firefox to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).
See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems