Strange Firefox issue

Got this strange issue with Firefox. I was customising it so that I save as much screen real estate as I can. I use the Tiny Menu addon and move everything UP to the menu bar (all the navigation items and the two folders in my bookmarks toolbar). I then turn off (uncheck) the Navigation and Bookmarks toolbars - leaving my with one menubar line and the tabs.

Now, odd thing is my two Bookmark folders appear only until I close and reopen FF. Then they disappear (i.e. minimize to a small blue arrow) until I "customise" the interface again - just clicking customise then closing the window that opens does the trick - no moving of buttons, etc.