Does as promised. Rated 4 out of 5 stars

Works as advertised, simply and cleanly changing the operating system reported by your browser. However, if you're looking for anonymity (not sure why else you would use it), this has pretty minimal effect. This is not the fault of the plugin, it's just that the OS doesn't actually convey all that much identifying info since there aren't all that many to choose from. Things like plugins and font lists are far more informative. Also, changing the OS reported by your browser makes a lot of download pages (including Firefox's) more difficult to use because they'll automatically present you with links to the OS they think you're using.

This review is for a previous version of the add-on (5.0.1-signed).