Start by checking that Firefox is up to date (from Help > About Firefox). You should have version 20.0.1. If you''ve got an older version, try simply updating it.
Then consider resetting Firefox (which fixes nearly all problems with it). If you choose to do so, you'll lose all of your add-ons, so go to Tools > Add-ons > Extensions first to make a list of anything you'll need to re-download. Then go to Help > Troubleshooting Information and click on 'Reset Firefox', at the top right.
Chris