From MozillaZine Knowledge Base

When JavaScript code runs for longer than a predefined amount of time, Firefox will display a dialog that says Warning: Unresponsive Script. This time is given by the settings dom.max_script_run_time and dom.max_chrome_script_run_time. Increasing the values of those settings will cause the warning to appear less often, but will defeat the purpose: to inform you of a problem with an extension or web site so you can stop the runaway script.

Finding the source of the problem

To determine what script is running too long, click the Stop Script button on the warning dialog, then go to Tools | Error Console. The most recent errors in the Error Console should identify the script causing the problem.

Extensions

If you get the Unresponsive Script Warning on many different web pages, the problem is most likely a recently installed extension.[1] You can try uninstalling extensions to fix the problem, or try creating a new profile.

Websites

If you get the Unresponsive Script Warning on a specific web page, it could be a problem with the JavaScript on the web page. You may want to contact the webmaster of the site to inform them of the problem, or you may wish to file a Tech Evangelism bug report. Steffen Rusitschka suggests using setTimeout on web pages that need to execute JavaScript for long periods of time.

Webroot Spy Sweeper

The Webroot Spy Sweeper application can cause an Unresponsive Script Warning on certain web pages. Try disabling or uninstalling the application, then clear cookies for the related websites. [2]