Video works in Firefox 25 as long as it isn't youtube or youtube based.
I tried using youtube in Firefox safe mode with all add ons and plugins disabled and it still doesn't work.
I installed the latest flash player and it didn't help. I also disabled hardware acceleration and that didn't help.
On my machine, I get an error has occured immediately when attempting to bring up a video.
On another machine in the network, it starts as if it is going to work and it doesn't move.