Safe mode in firefox allow users to temporarily disables the custom settings,themes and extensions.
To Run Mozilla Firefox in safe mode
Go to start menu->All Programs->Mozilla Firefox->Mozilla Firefox(Safe Mode).
In the Firefox Safe Mode dialog box, select the options and click Continue in safe mode button. In Safe Mode, your extensions and themes are disabled, and toolbar customizations are set to their defaults. But, these changes are not permanent. To start Firefox normally, exit Mozilla Firefox(Safe Mode) and start Mozilla Firefox. Now your extensions, themes, and settings will return to the state they were in
before you entered Safe Mode.