Are you using the Tab Utilities add-on? What you are describing is a known problem with that add-on and the latest nightly builds. The author is working on an update to fix it. To get Firefox working again you can disable all add-ons. To do that in the help menu select "Restart with add-ons disabled", then in the dialog box that appears select "Disable all add-on" and click "Make changes and restart". You can then use the add-ons manager to re-enable add-ons. If you have Tab Utilities you can re-enable all other add-ons. If you do not have Tab Utilities you will need to experiment with enabling add-ons to find what one is causing the problem.