Firebug no longer works Rated 2 out of 5 stars
It seems there is a conflict with Firebug: it does'nt work at all if HTML Notifications is installed.This review is for a previous version of the add-on (0.6.2.1-signed.1-signed).
Indeed. It has been fixed for new version, but mozilla validation takes some time. You might download an updated version at the project website http://code.google.com/p/ff-html5notifications/