[Ermenwyr]

It is? I've been trying to figure out how to get rid of this, but Googling various combinations of "remove space to left of tabs Firefox 57" has not yielded anything helpful.

[altern8tif]

May not be exactly what you're looking for, but under the Customize menu, you can toggle the Title Bar.

This adds a macOS-style title bar and flushes the tabs to the left of the window.

[kuschku]

Menu -> Customize has a few customization options at the bottom, the one you want is called "drag space".

[Sylos]

I think, it should be Configurable under Customize and then one of the checkboxes at the bottom.