This problem was driving me nuts for a long time. All kinds of videos would play ok, except in Facebook on Firefox. I reinstalled Firefox and Adobe Flash, to no avail. It turns out that ADBLOCK was mistaking the facebook videos for advertisements. Go to TOOLS, click on ADBLOCK PLUS, click on filter preferences and click on add filter. Then enter the following as the filter name: @@||facebook.com^$document . This will stop ADBLOCK from blocking Facebook videos. Hope this helps and it doesn't take you as long to fix this problem as it did me!