how do I re-enable "unresponsive script" warning in Firefox?

Open the page about:config in a new tab, and reset the pref dom.max_script_run_time: http://kb.mozillazine.org/Dom.max_script_run_time

The "Don't ask me again" checkbox only applies to the currently running script.

If you haven't ticked it, the warning will pop up approximately every dom.max_script_run_time seconds while the script is still running.

Simply refresh the page (or run another script) and you should get the warning again if your script is taking too long.

Go to the about:config page.

In my Firefox there are now three script variables that are probably changed when checking the checkbox:

dom.max_child_script_run_time
dom.max_chrome_script_run_time
dom.max_script_run_time

Just type script_run in the search box and it shows just these three. Right-click each one of them that's bold and choose 'Reset'.