This is exactly the same as User Agent RG 1.0 - No Restart by justin Ƨullivan.
The only difference is that I have updated the available Firefox strings to include all main versions up to 18.0
For the time being, I do not intend to update the user-agents of any other browser, as my focus is on Firefox and its siblings.
But if you want to do that yourself, it should be a good & easy hacking lesson for you :)
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Before you install this updated extension, please uninstall the original User Agent RG first. This is to avoid possible conflicts.