In Safe Mode, all of your extensions will be temporarily disabled and the default theme is used. This will help determine whether an add-on is causing your problem.

Close down Firefox completely: At the top of the Firefox window, click the File menu, and select the Exit menu item.Close down Firefox completely: On the menu bar, click the Firefox menu, and select the Quit Firefox menu item.Close down Firefox completely: At the top of the Firefox window, click the File menu, and select the Quit menu item.
In Windows, click Start, open the All Programs list, and navigate to the Mozilla Firefox folder. In the Mozilla Firefox folder, select Mozilla Firefox (Safe Mode).
3, Mac OS:
Hold down the Option key, as you start Firefox.
2.0, Mac OS:
Go to the Utilities directory (in the Applications folder) and open Terminal, then run:/Applications/Firefox.app/Contents/MacOS/firefox -safe-mode
Go to your Terminal and run:firefox -safe-mode
Firefox should start up with a Firefox Safe Mode dialog.

Note: You can also start Firefox in Safe Mode by clicking Start, selecting Run (or use the Start Search box in Windows Vista) and then entering the following in the text field:
firefox -safe-mode
When the Firefox Safe Mode window appears, press the button Continue in Safe Mode.

If the problem persists in Safe Mode, your problem is not being caused by an extension or theme. It could be caused by a plugin - plugins are not disabled in Safe Mode. See Troubleshooting plugins.

If the problem is not present in Safe Mode:

One of your Firefox configuration files may be corrupt. Reset your toolbars and controls to see if that solves your problem.
The theme you're using or one of your extensions could be the causing the problem. Continue with this article to troubleshoot.
If you have been using a new theme, switch to the default theme and restart Firefox to see if the theme was causing the problem.

Click on the Tools menu, and select Add-ons. The Add-ons window will appear.
In the Add-ons window, click the Themes icon.
Select the default theme, then click the Use Theme button, to make Firefox switch to that theme.
Close Firefox, then start it up again.
When Firefox restarts, the default theme is in use. If the problem goes away, your theme was causing the problem.