The dom.max_script_run_time is an integer that is the number of seconds to wait before prompting that a script is taking too long to respond. I found this information over at Itchy Hands.
To fix it, youll need to access the hidden configuration page in Firefox. Just type about:config in the address bar and youll get a page full of hidden configurations which you can tweak till your hearts delight.
In the 'Filter' box, type dom.max_script_run_time. This will narrow the options till only the dom.max_script_run_time option remains. Right-click it and choose Modify. A box pops up. Change the number to something bigger like 20. This is the maximum time a script can run before Firefox considers it 'unresponsive'.
If you cant find the string in the about:config page, create it by right-clicking anywhere and then choose New—> Integer and enter dom.max_script_run_time. When prompted for the value, use 20.
So far Ive not had anymore unresponsive script warnings, so I guessed that fixed that problem! However some extensions may also cause similar problems. Hope this helps.