Rated 5 out of 5 stars

Everyone wants a 4.x update, but it works just fine (for me) with 4.0.1. On a mac, here's what to do. First, download the JAR file. I did it by going to https://addons.mozilla.org/en-us/firefox/addon/whitehart/ using Google Chrome and clicking "download anyway". Then open "Terminal" from the Utilities folder in your Applications folder and run the following:

cd ~/Downloads
mkdir wht
mv whitehart-3.6.7-fx.jar wht/
cd wht/
jar xf whitehart-3.6.7-fx.jar
mv install.rdf install.rdf.OLD
sed -e "s/3.6.\*/4.0.*/" install.rdf.OLD > install.rdf
rm install.rdf.OLD whitehart-3.6.7-fx.jar ; jar cf whitehart-3.6.7-fx.jar *

Then go into add-ons in Firefox, choose from the drop down in the upper right to install Add-on from file, and select your new JAR file (in the wht directory of Downloads). Works great for me.