Don't waste your time, use the better option... Rated 1 out of 5 stars

This thing doesn't provide any functionality that can't be achieved by entering about:config in the address bar and filtering by general.useragent.extra.firefox There is a much better addon called HeaderControl. HeaderControl provides domain based user agent switching. (something that has been requested of User Agent Switcher time and time again) This means you can set the addon to send specific user agents to specific websites. Finally!