Occasionally we get reports from Firefox users that videos on YouTube start with several seconds of a black screen, after which they play normally. Pausing AdBlock shows an ad and then the video plays instantly.
Poking around the web a bit, we found a post in a Firefox forum that suggested this is related to Google's decision to change the default YouTube video player in Firefox from Flash to HTML5. The workaround is to install an extension that forces the video player back to Flash. The extension is Disable Youtube™ HTML5 Player. Give that a try and you should be all set!