> The tabs don't hang in the top left corner but there's some space.

Which is configurable.

I totally get your point, but please, Mozilla, make the default configuration great or people will stick with a browser who is owned by an evil company. :)

Btw: What I mean is the space left from the first tab, not the "drag space" on top.

> Btw: What I mean is the space left from the first tab, not the "drag space" on top.

The drag space should enable/disable both, the space left of the first tab, and above the tabs.

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.

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.

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

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