Very simple extension, does exactly what it says and nothing more. If you don't believe me, audit the code yourself. My only complaint is the lack of a whitelist.

Compatible with Waterfox! This add-on has absolutely no options, no icons, nothing! It works completely automatic, selecting a random agent string for each domain visited by the user. Unfortunately, there is no way to select/edit available agent strings, whitelist/blacklist domains or anything at all.

It seems to work, but on some sites such as addons.mozilla.org, it says I require newer version of firefox to use a certain addon. This addon would be even better if it also allows custom agent selection to avoid problems like these.