A fast and easy solution is:
1 At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP), and then click Add-ons. The Add-ons Manager tab will open.
2. In the Add-ons Manager tab, select the Extensions or Appearance or Plugins panel.
3. Select the add-on you wish to disable.
4. Click the Disable button.
5. Click Restart now if it pops up. Your tabs will be saved and restored after the restart.