The latest release version from the author's Github page works great. It's safe and practically the same thing. If you install it from there, it's called "Wikipedia Peek - DEV" in the FF addons manager. (Don't forget to disable or remove the regular one you downloaded from AMO.) Not sure why this isn't updated on AMO, but as long as the dev version works, it's a very helpful extension.

One thing I noticed is that this works on other sites than Wikipedia by default, but you can actually just remove the entry in the extension's options. (The pages/tabs need to be reloaded after this for it to work, or rather stop working on them.) It also adds a button, with which you can temporarily "disable" it when it gets in the way, so there's that.