Close YouTube with Ctrl+W / enable browser shortcuts in Firefox Flash windows
A Firefox bug is responsible for browser shortcuts becoming useless when you play a movie on YouTube or on other sites with flash content.
If your keyboard includes a Start (Win) button, the one with the Windows flag, just press this button twice, one time to open the Start menu, and another one to close it.
This way you shift focus from Firefox entirely and immediately you give it back – but this time the focus is indeed on firefox, not on the flash movie, which means that now all browser shortcuts work — you can switch tabs, close the tab with the movie, go to the address bar, to the search bar, anything you like!
If your keyboard lacks a Start button, then just press Alt+Tab twice, switching to some other application and then coming back to Firefox. The secret is to steal and give back focus to Firefox in the easiest possible way. If this won’t work on some page, you’ll have to use your mouse.