Following the latest update of the Firefox browser (to v16.0.1), I've been having problems with it hanging constantly.
The solution was to reset it to its original state. This means that custom add-ons, themes etc are removed and saved to a folder on your desktop so you can recover any that are important (Mozilla advises you delete this folder when it is no longer required as it contains personal data). So far this solution seems to have worked.
You'll find the button to click to reset the browser under Help>Troubleshooting Information.