Hopefully something here will be useful

Updating User Agent Switcher breaks Firefox

Twice I've run into a problem where updating the User Agent Switcher extension for Firefox has broken the browser. When you try to start the browser none of the buttons work and your left with a pretty worthless window. Apparently the User Agent Switcher extension would prefer to be uninstalled and reinstalled manually rather than updated. Ok, so first you need to get into Firefox. Fortunately Firefox can be started in safe mode which does not load any extensions. To do this run the Firefox executable manually (or edit the shortcut) with the "-safe-mode" argument to it. Once in Firefox you can just uninstall the extension and then proceed to install it again.

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