Modified for 4.0 Rated 5 out of 5 stars

Ok everyone here is a modified version that will work with Firefox 4.0
http://www.megaupload.com/?d=Y5LAY623

I modified it via the instructions in gtmb's comment and it works well. Just download then drag and drop into the browser and hit "Install"

This review is for a previous version of the add-on (0.9.1). 

Updated

Thanks for the assist. I've recently updated it to support 4.0.*. I've also added it to GitHub. The CheckFox project (and the XPI most users would be interested in) can be found here: https://github.com/seanleblanc/CheckFox

For those willing to wait, it's been submitted to Mozilla Add-Ons, as well.

Cheers.