Your toolbars become vertically Drag and Droppable !
Install this extension, and you will be able to chose the vertical order of your toolbars simply using "Drag and Drop".
When you want to modify the order of your toolbars, use the context menu (right click) on anytoolbar.
Check the 'Unlock toolbars' : you can drag and drop them on each other !
When you uncheck the 'Unlock toolbars' option, their positions are saved, and will be restored each time you run Firefox.
Suggestions and bug reports welcome AND answered:
WHAT IT CAN'T DO:
You will not be able to merge toolbars, or add one at the bottom of another one. PLEASE DO NOT COMPLAIN ABOUT IT, I never pretended it could do it :)
A Firefox built-in feature allows you to move buttons from a toolbar to another (if toolbars were properly designed). Use the "Customize" menu option for this purpose.