There are a few items that are locked from customization from the Customize Toolbar... screen:
I've been told that the space on the left is there so that people have a place to grab the window. However, there are, by default, two flexible spaces added around the address bar that can be grabbed. Strangely, the space disappears when the window is maximized. This doesn't make much sense: The space could still be used to grab the window and restore it to its original size... If it were still there. Changing the appearance and functionality of the tab bar when the window is maximized doesn't serve any purpose and is a bit confusing.
In any event, I'm requesting that the inflexible space be removed. If the developers want an easy way for users to grab the window in addition to the two already available by default flexible spaces, replace it with a flexible space. This way, users who don't want a chunk of their tab bar to be blank and useless can simply remove it.
Sidenote on the FF View button: Perhaps the icon can be modified a bit so it's not simply the FF icon but contains an element or elements that indicate its function.