Firefox multi row tab bar?
TabMixPlus allows multi-row tab bars.
It will allow you to choose the maximum number of rows you want it to show.
Not exactly the answer to your question, but I find Tree Style Tab very useful. Gain some height and order, at cost of some width.
Tab Kit - Tab grouping, vertical tab tree, multi-rows, and various tweaks for power users.
Tutorial: Customize Tab Behavior in Firefox with Tab Kit