After installing CTR in Waterfox 56, I'm unable to add spaces between the buttons on the toolbar. Dragging a space from the Customize menu takes it up there, but is doesn't remain in place. Same goes for flexible spaces.
In the top half of the attached screenshot a space has been dragged to the spot indicated by the red arrow, but after releasing the mouse button, the space has been removed and has reappeared in the Customize panel.

Here's what it looks like after closing the Customize menu.
