That sometimes happens randomly on videos, but if it is every video you try to watch, my bet is on Javascript not being enabled. Go to Tools > Options > Content > Enable JavaScript, and make sure it is checked. If not that, then update your Flash player, and if not that, update Firefox.
Also, make sure you are not running Firefox in “No Add-Ons” mode.
I tried all that. Still no luck. I checked on Google and this now seems to be a major problem. Many others can’t watch videos via FireFox.