I'm running OSX 10.8.5 on an iMac 27", 2Gb 1Tb disk with Firefox 25.0 and Flash 11.8.800.168.
I just tried a Vimeo URL @ http://vimeo.com/74066023 and it ran for 2 seconds and stopped. I could restart it by manipulating the slider at the bottom, but it would only run for two second before stopping again.
So I tried a Youtube video @ https://www.youtube.com/watch?v=n93kBM3X2Js This stops after 8 seconds., can be restarted by aligning it to any point and it will restart - but only for 8 seconds.
This si a new phenomenon as far as I'm concerned, but I don't watch a lot of video content.
It works fine with Chrome (which doesn't use Flash, I believe). Is there some setting I need to make it work?
1. Firefox 25 is the beta so, like ALL betas, you can expect some things to not work right.
2. Chrome has its own embedded Flash Player (called "PepperFlash").
Have you done a "clean install"?