I can't explain what they were thinking when they hid the button but here's the thing: In firefox 6 you can right-click on the toolbar and select "customize" to access a panel with more buttons that can be added to the firefox UI. One of the available buttons is the fullscreen button. you can drag it into the top right corner of the window. It will be ''almost'' identical to the native button (but not quite in the right spot) Not ideal but it's better than nothing.