If you are using Flash with YouTube, you need to allow scripts. Not sure if you need them for HTML5. To give scripts more time, I changed these settings in about:config: dom.max_chrome_script_run_time -> 30 dom.max_script_run_time -> 20

I get fewer "unresponsive script" error messages that way.

Another common cause of unresponsive script errors on Windows 7 is the protected mode feature of the Flash player plugin. That feature has security benefits, but seems to have serious compatibility issues on some systems. There is a hidden setting to disable it; I suggest trying this for a day to see whether it helps:

(A) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(B) In the search box above the list, type or paste flash and pause while the list is filtered

(C) Double-click the dom.ipc.plugins.flash.disable-protected-mode preference to switch its value from false to true.

This might not take effect until all Flash has been unloaded for a few minutes, or you close Firefox.

Any difference?

Next time you get script errors, write down Exactly what the message says. Then post it here.