Hello,

One of your extensions may be causing this problem. Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thank you.

EDIT: since this morning, Chrome also has the same problem, so this is no longer a Firefox issue, but most likely, a problem at my end.

I have been having the same issue since updating to the latest release of Flash Player. Following the safe-mode instructions, I have identified the issue to be AdBlock Plus 2.5.1. Whether or not this is down to the add-on, YouTube or FireFox, it seems to be blocking a lot of Flash Player content at the moment.

Try the ABP forum.
http://adblockplus.org/forum/