Right click toolbar, "Customize…" Rated 4 out of 5 stars
If you install this extension, make sure you right-click the toolbar, select "Customize…", and drag the "Firefox" menu icon to your toolbar.
(Ideally, this extension should add the icon to your toolbar automatically.)
…but, overall, it's a great addition to make Firefox feel more at home when using GNOME! (:
It fixed in 2.1