If it is supposed to find leftover stuff from already uninstalled extensions it didn't find any on my machines.
Two main functionalities: 1. Notify you about the left preferences upon an extension removal 2. Allow you to clean your Firefox profile by removal old unused preferences left by your old extensions. To access this, go to the options page. Nice find. I have tried a lot of add-ons on my current profile, so 2. looks interesting. Will run unsandboxed and give it a go.
Why does the HTTPS Everywhere icon change position in Firefox when it updates? No other extension does that. Yes, I know it is easy enough to move it back into place.