I have been using this addon for quite some time, and it is perfect for small screen, or just for a nicer look. And on both Firefox and Thunderbird!
The addon will also still be useful with Firefox 3.6, since the developers had the bad idea to decide that the menubar must be shown if one added anything to it. Which is, of course, what we all did.
Let's hope for an update!
Please help to vote https://bugzilla.mozilla.org/show_bug.cgi?id=538580