@siffemoz
The style should stop working after commenting out the code and restarting the app.
Try exiting the app and using the -purgecaches command line option.
i.e.
thunderbird.exe -purgecaches
ThunderbirdPortable.exe -purgecaches
More info
http://developer.mozilla.org/docs/Extensions/Updating_extensions_for_Firefox_4#Caching