I am having the same issue, ever since FF8 and it is really annoying, especially on older/slower computers where pausing the video & allowing it to buffer for a minute prior to watching it is essential. The videos still buffer in Chrome & IE, but just not in FF - even in safe mode/with all add-ons disabled. I've cleared my cache, all my plugins are up-to-date, nothing has fixed this issue. I there are multiple posts on this topic and I have yet to come across one with a reply yet alone one with a solution or cause.

I have the same problem with FF 9.0 and previously 5.0, on a brand new Windows 7 PC. No problem with IE. No useful answer from this group yet.