There are hundreds of these add-ons, they have one goal, same names, similar functionality, and I even found an outdated rip off of this one.
Why choose this over the others? If you want to easily maintain your own list and you don't need an outdated list of 1000s of deprecated browsers, or a pretty interface with big flashy icons. If you want to spoof an agent and have it stick on browser restarts or spoof the agent for a specified website and have it stick or spoof random agents at an interval than this add-on is for you. I have set this up for an old lady so it's not only made for "hackers".
Developer: The interface text size and spacing could at the users discretion be decreased so we can fit more items into view, I'm using the default list and it takes up a lot of room. The icon is ugly! It's identifiable in that I look for the ugly icon. It looks like a box with a small dot in the center, consider increasing the empashis of the 3 "heads?" or making a new icon with out the noise in the background. minimalism is the current trend for a good reason.