"Flexible Space" creates a gap between icons which is too large for many purposes (see screenshot, the large space looks rather ugly). I suggest allowing smaller spaces, about the width of one icon, to group toolbar icons. Another way to group icons (maybe in addition to smaller spaces) could be vertical separators, as can be seen between History icon and the Toolbar Menu in the attached screenshot.

Icons in Firefox 56, with smaller spaces, thanks to "Classic Theme Restorer" addon

We are aware that this would be a possibility, but it likely won't happen for 57.