I experience weird behaviors when customizing the Navigation toolbar (and other FF4 toolbars as I recall): Every time I move or remove the last item (the one on the most right-hand side) in the Navigation toolbar, the following occurs: - The New Tab "+" button on the Tab bar disappears - The whole Navigation toolbar is blanked out - The menu bar (if displayed) is grayed out - All the toolbars disabled in View stay visible when closing Customize and Customize itself is grayed out, not offering the chance to fix this problem. If I close and reopen Firefox 4.0, the menu and toolbar appear and the Customize feature is selectable, except that the Navigation toolbar is blank, so I have to redo all the customization again. On another note, when customizing toolbars, there is no visual indication telling you which toolbar it is you are modifying. Since the height of the toolbars is very small, there should probably be a tooltip or some visual aid (the toolbar name) showing if you are dropping items on the Navigation or another toolbar. See 'more system details' for steps to repro.