Gnome 2.28 changes the button-images setting to not show icons in buttons by default. Standard GTK+ dialogs that are used by firefox (file chooser, print dialog, etc) follow this setting, but custom dialogs such as File > Import... Edit > Preferences View > Toolbars > Customize... View > Character Encoding > Customize List... Bookmarks > Bookmark All Tabs... don't.
Jan, it's fixed in rawhide, right?
Fixed in rawhide.