Unresponsive Script Warning
From MozillaZine Knowledge Base
Complete Error Message:
Anything that causes the mozilla application to slow down can cause this warning to be issued. The slowdown may be caused by an extension, but it might also be a web page, another program, or resource utilization (CPU, memory, disk IO) at levels high enough to impact one or more application.
Finding the source of the problem
Click the Stop Script button on the warning dialog, then depending on your version, go to Tools > (Web Development) > Error Console. The most recent errors in the Error Console will identify the script that was running. But note, in some cases the add-on/extension causing the problem is not the one identified in the messages.
Unresponsive Script is often caused by a recently installed extension, or an update to the extension or application thus causing some incompatibility between the two.
Disabling extensions or rebuilding a new profile
Several techniques can be used to identify bad extensions. A quick test is to start in Safe mode. If it works better in safe mode, then start in normal mode and try disabling/uninstalling extensions to fix the problem - either one at a time, or in groups. To do this in groups, disable half of the extensions and if failure continues, disable the other half. Enable and disable addons in successively smaller groups until reduced to just one extension that fails. Or try creating a new profile. If the new Profile works correctly, then you can migrate specific files:  and reinstall extensions.
Some extensions reported having Unresponsive Script Errors
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. 
Finding similar problems
Finding others with similar problems may find something that helps.