How To Remove “Unresponsive Script” Warnings in Firefox

You need to raise the default script execution time. Here’s how:

Type “about:config” in your Firefox address box.
Below the address box you’ll now see another long, empty rectangle – the Filter box. Here, type “dom.max_script_run_time”. Or just copy and paste it from here to there – but without the quotes.
Right-click on this row, and select Modify.
Replace “5” with “30” to raise the default maximum to 30 seconds.[source]