If you use this great add-on, use the following link to reinstall a patched version automatically. Clicking this link will then make firefox ask if you want to install. Click yes and that’s about it. Firefox will have to be restarted for the change to take place.
https://github.com/downloads/athomas/mozilla-firefox-addons/gmail_manager-0.6.4.1.4-fx+sm.xpi
If you want to do it manually, see how to edit and patch it yourself in this review here (advanced users)
