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Safe Mode runs Firefox with your add-ons disabled. Follow these steps to start Safe Mode:  Select "Mozilla Firefox (Safe Mode)".  Read the Troubleshooting Extensions and Themes article on the Firefox Support website for more information.
Sometimes add-ons for Firefox (extensions or themes) can cause problems. Make sure Firefox is closed (open the "File" menu, then select "Exit")  Click the Start button, open the All Programs list, and navigate to the Mozilla Firefox folder. When the Safe Mode dialog box appears, click "Continue in Safe Mode"  If the problem does not occur in Safe Mode, and extension or theme is likely the cause.