Update: have just discovered that it works fine if the navigation toolbar (address bar etc) is disabled. Of course, one can't do much on the internet without the nav toolbar, so it is not a practical solution.

Do you have that problem when running in the Firefox SafeMode?
A troubleshooting mode.
1.You can open the Firefox 4.0 SafeMode by holidng the Shft key when you use the Firefox desktop or Start menu shortcut.
2. Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut to open it again.

If not, see this:
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Thank you. I tried safe mode, and the problem did not occur any longer. So it has to be one of my addons, and I am not using that many.

What I find quite intriguing is that I am now back in normal mode, and the problem has disappeared. Go figure...

Ctrl+T would not work but Firefox Safe Mode fixed the problem. Thank you.

I've noticed this problem on websites powered by flash...Youtube being the worst. Disabling the flash plugin would fix the solution, but be impractical.