Firefox 4 has a tabs on top feature, but it doesn't really work if you have customized toolbar.
For example: I've got all my navigation buttons and address and search fields on the file "Menu Bar" toolbar, When you select "Tabs on top", the tabs move above the bookmark toolbar, which is pointless. It looks like this:
navigation, address, search, menu tabs toolbar bookmarks toolbar
I'd prefer something like this:
bookmarks toolbar tabs toolbar navigation, address, search, menu