I have that problem as well. Watching on YouTube works, so that is acceptable as a workaround. That problem does not occur with Opera or Safari.

Make sure that you aren't blocking cookies.

Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Press and hold Shift and left-click the Reload button.
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (MAC)

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I doubt that it is a cookie issue. I allow cookies for the forum where I usually see the embedded videos from YouTube, but not for YouTube. That works fine with Opera, not with Firefox, and I use the same cookie settings for the same sites with both.

It is Adblock Plus causing the problem. If I disable it on the page where the embedded video is, the video plays. I would rather use a workaround than disable Adblock Plus.

Yeah, it's Adblock Plus for me too, even if it's not blocking anything on the page. If I disable it for the page in question, the video loads. Adjusting my Adblock filter preferences seems to have fixed it.

What did you change? I presume there was some URL that it normally blocks that has to be unblocked?

Tools>Add-ons>Adblock Plus, then the Filter Preferences button. From there, the Custom Filters tab, and in the Ad Blocking Rules category I had youtube.com/crossdomain.xml entries. After unselecting those and saving, the problem seems to have been resolved.

Got it. Thanks, it works.

If you have the Adblock Plus toolbar button on the main toolbar or on the addon bar, you do not have to open the add-ons window to change the filter preferences.  All you need to do is right-click the button and choose Filter Preferences.

I'm having this problem too, but its not adblock... I disabled it, but still doesn't work.