Very powerful, with a very deep selection of user agents that you can easily sort through based on OS and browser. Blacklist, Whitelist, and Custom filtering is available in the addon options, though curiously you have to choose between the three instead of having a combination of them. No way to bulk add user agents is the only real downside to this addon, but when it comes with such a large selection to begin with this won't inconvenience many.

I do recommend the Dev add a shortcut from the popup to get to the addon settings though.

Needs an option to export custom strings from an XML, so it won't need updating once in a while.
Overall, looks pretty solid.

Wow it is easy to use and portable too. thank you!!