Modified for 4.0 Rated 5 out of 5 stars
Ok everyone here is a modified version that will work with Firefox 4.0
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"
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.