Both 'not responding' and 'unresponsive script' problems with Firefox usually come about when
something else on your computer is hogging nearly all of its resources, leaving Firefox with nothing to work with. The usual culprit is an antivirus program that's trying to update itself. (Windows Security Essentials is notorious for it but other security programs can also cause the problem).
See my post here for a suggested way forward:
http://www.theanswerbank.co.uk/Technology/Question1492287.html
(That post refers to 'unresponsive script' problems but it applies just as much to 'not responding' errors).
You could also try resetting Firefox to its default state. To do so, first go to 'Add-ons' (via the three horizontal lines in the top right corner). Select 'Extensions' and note down the names of any extensions which you wish to retain (because you're about to delete them). Then go to 'Help' (via the three lines and the little button with a question mark in it) and 'Troubleshooting Information'. Click on 'Refresh Firefox'. Then reinstall any required extensions (although it might be worth trying Firefox without those extensions first, to see if they're contributing to the problem).