This seems to have been a part of bug 667480. I don't think these buttons need that style?

They do; they're on a bottom bar (status bar) that looks like a toolbar.

Then maybe this shouldn't use -moz-appearance:statusbar in the first place? http://mxr.mozilla.org/mozilla-central/source/toolkit/themes/osx/global/wizard.css doesn't seem to set this either.

I don't know, maybe the toolkit wizard should start using -moz-appearance:statusbar, too? All I know is that it looks much better with the statusbar + toolbarbutton appearance, and that this look is what UX asked for in bug 480178 comment 18.