One feature a lot of people miss - and someone told me about it again only a few days ago - is the ability to move toolbars and agregate them on a single toolbar line. If you're a bit familiar with Microsoft products, that's clearly one area of strength in their UI customizability (if you except the lame customizability of Internet Explorer...). Here's how we could add it to Firefox at a quite low cost (hey, given that chrome.manifest even allows to override a single file in a jar, it could even become an extension!):

  • on load of the browser's xul window, browse all toolbars ; if a toolbar contains no toolbargrabber, insert a toolbargrabber as first item in the toolbar
  • change toolbar.xml - or AddNewToolbar() in customizeToolbar.js - to make it insert a toolbargrabber element in all new toolbars created by AddNewToolbar()
  • write an xbl for toolbargrabber so
    • the element is drag'n'droppable
      • if it dropped on the browser's menubar, the toolbar will be moved before the first toolbar in the browser
      • if it is dropped on the elements in an existing toolbar, it will be moved right after that toolbar
      • it is dropped below the last toolbar, it will moved  as the last toolbar just above the content area
      • if its dropped on the toolbar itself or on a trailing flexing spacer, the toolbargrabber and all the toolbar items after it will be added to the target toolbar after a spacer of a specified width computed from the event location so the toolbargrabber can be positioned precisely ; the toolbargrabber will keep track of the original toolbar's name ; the original toolbar, now empty, will be dropped
      • if a toolbar is created from a toolbargrabber not being the first toolbar item of the toolbar, it acquires the name of the toolbar the toolgrabber preserved
    • a double-click on a toolbargrabber that is not the first item in its toolbar restores the toolbar with the name stored by the toolbargrabber right after the current toolbar parent
  • change customizeToolbar.js so it's not possible to move a toolbar item before a toolbargrabber is that one if the first toolbar item in the toolbar