Interesting idea Rated 3 out of 5 stars
Interesting idea. But when you visit a website with an embedded sun java applet and you are using an IE user agent, the applet and the browser will crash (at least that happens with my FF).
Also i miss an option where the user can choose a (constant) default language. Mine was altered to "de" after installing the extension.
At long last, it would be cool if there were an option where the user can manage (add/remove) user agent strings.
some of your points, I will certainly implement in the near future. but at the moment i have to little time.