THIS FEATURE IS NATIVE TO WIKIPEDIA NOW. You can enable or disable it on the wikipedia site. For logged-in users, Page Previews may be re-enabled from user settings under the section titled "Appearance". For logged-out users, Page Previews may be re-enabled by selecting the "Enable Previews" link at the bottom of the page. Search "turn on page previews in wikipedia" there is also an advanced preview option called "navigation pop up" if you are signed in on wikipedia. This extension worked very well but as far as I know it is no longer updated so I use the native option on the site.

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.

Rather useful addon. Thank you!

Great extension!
Thanks!!!

the ability to look ahead gives you you the opportunity to get an "I thought that was what it meant" or a "Duh -- Glad I looked that up" without having to leave the primary page you were on. 'Cool Previews; enabled the pop-up,when desired, of whatever page any link pointed to in a user-selected size box. Anyone know of a modern version of this extension?

very comfortably

Great extension, but the one built in to Wikipedia is available to everyone now, and this isn't needed anymore.

Simple to use and very reliable.

Good