On Firefox, the empty space that appears on either side of the address bar is known as “Flexible Space,” which allows you to add extra space to separate groups of items, or to center or make the address and search bar smaller by adding multiple flexible spaces.
If you want to add or remove empty space in the toolbar, it’s a very easy task on Firefox.
Right-click the toolbar area, and select Customize.
Flexible Space is represented as rectangle, simply drag and drop a rectangle in or out of the toolbar to add or remove empty spaces.
Click the Done button.
Alongside Flexible Space, note that you can add and remove virtually any items that appear in the toolbar, including the search bar, home, history, full screen, and many more.