It's almost certainly NOT a problem with Firefox itself.
Firefox 'not responding' problems are far more usually associated with something else on your computer hogging all of its resources, leaving insufficient for Firefox to work with. It nearly always turns out to be the updating element of a computer's security software which is causing the problem, so the fix is to uninstall your anti-virus program and then reinstall it (or install something better). The problem is particularly common with Microsoft's own security software (MSE, Defender, etc) but I've also experienced it with AVG and others here have noticed the same thing with Norton.
However if you want to restore Firefox back to its 'out of the box' state (but with your bookmarks, etc preserved) it's dead easy:
First ensure that you've got 'File', 'Edit', etc across the top of your screen. (If you haven't, right-click at the top of your screen and put a tick against 'Menu bar')
Then go to Tools > Add-ons > Extensions and note down the names of any extensions you're using and wish to preserve (because you're about to delete them).
Next go to Help > Troubleshooting Information and click on 'Refresh Firefox'.
Lastly go back to Tools > Add-ons > Extensions and reinstall anything you need there.