navbar/auto-hide.css not working properly in Firefox 58

[dannycolin]

Hi

I just updated to FF58 this morning and i found out that there's a extra gray bar when the navbar is hidden.

https://imgur.com/a/HLQgA

It seems that the variable --tab-toolbar-navbar-overlap is undefined by default. So I've changed the line 20.

Before :
--tabbar-height: calc(var(--tab-min-height) + var(--space-above-tabbar) - var(--tab-toolbar-navbar-overlap));

After :
--tabbar-height: calc(var(--tab-min-height) + var(--space-above-tabbar));

That workaround is causing a minor glitch. When I hover the tabbar the webpage seems to go up by 1px.

EDIT
[2017-11-23] See #67