As much as I like Firefox, beginning with version 29 and just now version 30, tabs only show up on top — whereas in previous versions one could enable or disable that feature.
(I've already lodged a complaint with Mozilla, but I'm not sanguine that anything will come of it.)
If there is a (surreptitious) way of achieving tabs below the navigation (and bookmark) toolbar in the versions since v.28, I'd like to know how (such as a toggle in about:config).