• Firefox 3.0.4
• Adblock 0.5.3.043
• Web Developer 1.1.6
• QuickTime 7.5.5

The above combination (and variations of it) disables video content from playing — tested on an iMac and MacBook Pro using this page.

If you disable Adblock, it will attempt to load the content, but will only show a QuickTime logo with a question mark.

If you disable Adblock and Web Developer, the content will load and play.
I also have the following extensions installed, but none of them appeared to effect the content loading or playing.

• Download Statusbar 0.9.6.3
• Firebug 1.2.1
• PDF Download 2.0.0.0
• Steep and Cheap Watcher
• StumbleUpon 3.26