Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

You can also try "Reset all user preferences to Firefox defaults" on the Safe mode start window.

In Firefox 4 you can use one of these to start in Safe mode:

• Help > Restart with Add-ons Disabled
• Hold down the Shift key while double clicking the Firefox desktop shortcut (Windows)

Hi cor-el, your suggestion solved the problem. The video loads in safe mode. But how do i fix the cause of the problem when i restart to normal mode?

What extension could be the problem?

How do i narrow the trouble extension or plugin in normal mode?

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

In Firefox 4 Safe mode disables extensions and disables hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

If disabling hardware acceleration works then check if there is an update available for your graphics display driver.

I had the same problem. I tried all the solutions above from cor-el without any luck.

But after clearing offline storage, youtube started working again.

Tool -> Options -> Advanced -> Network -> Clean Now (under Offline Storage)