Are you running the videos in full screen mode?

If that is the case then you get a play progression bat and probably other content if you hover the mouse.
You can usually press the escape key to exit full screen mode.

You can also try to clear the cache and the (YouTube) cookies from sites that cause problems.

"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > Cookies: "Show Cookies"

Thanks cor-el. Yes, I am viewing in full-screen mode, but that's the way I've viewed videos on YouTube for years, in Firefox, and I've never had the problem of the progression bar and other content just drifting up and down every few seconds. I am not hovering the mouse.

I regularly clear my caches and cookies, and that hasn't helped. This has happened only since I updated to Firefox 18.0.1. It seems as if every new version of Firefox has some glitch, which never used to be the case.

Thanks for your ideas, anyhow.

You can check for problems caused by recent Flash updates and try these:

• disable a possible RealPlayer Browser Record Plugin extension for Firefox and update the RealPlayer if installed
• disable protected mode in Flash 11.3 and later
• disable hardware acceleration in the Flash plugin

• http://kb.mozillazine.org/Flash#Troubleshooting

I've just downloaded the latest Flash, so will see what happens with that.

Not sure if I feel confident enough to try the other things--if some new problem appeared, I would not know what to attribute it to or exactly how to backtrack and redo any disabling and so forth.

BTW, a different question: below is a screen shot of my applications folder. I'm looking at all of the Flash icons, the Firefox icons, and the little "install flash player" gray icons--do I have to keep all of those? What would happen if I dragged all but the most recent to Trash and emptied them?