The add-on was great, but support has been dropped and as of recently seems to have been completely broken on Wikipedia. Fortunately, Wikipedia has a native equivalent now. Unfortunately, the native equivalent sucks by comparison, as not the entire summary is displayed. For logged-in users, under Preferences > Gadgets, you can enable navigation popups which is somewhat better than the default but still inferior to this extension, were it still functional.

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 This extension worked very well but as far as I know it is no longer updated so I use the native option on the site.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

This add-on was great for sites like Wikipedia and Fandom, and it still is. But I wish you could update the animation so that it looks fancier as well as give users a way to manually set the themes through the add-on settings page.
Also, since Wikipedia now has it's own built-in link preview feature, this add-on kind of interferes with it. The add-on is still really useful on websites like Wikia, but it would be nice if there is a way to disable this add-on for specific websites.

Positiva.