Open Options > Advanced, then click on the Config Editor button. Now read what it says inside the active window title. So even without a description of this extension, you should already know what about:config is. If you aren't familiar with the contents of the Options panel, then perhaps you shouldn't be using Thunderbird at all. It irks me to no end that you gave this extension a 1-star rating just because you haven't familiarized yourself with Thunderbird's built-in features.
It's ridiculous to "rate" something that you don't know how to use, what it does, or why you might want to even install it. This does exactly what is supposed to do, works perfectly with Thunderbird 2.0, and deserves every star I've given it: *****