New tweak: hide tab bar

[chris-morgan]

As a user of a sidebar tabs extension, I don’t want the native tab bar.

[Timvde]

How does this work on all platforms, and with/without title bar? There's an open PR that also hides tabs here, you might want to look into it and join the conversation.

[Timvde]

So, I merged the other PR. Currently, hiding tabs is very simple: https://github.com/Timvde/UserChrome-Tweaks/blob/master/tabs/hide-tabs-linux-windows-titlebar.css

What do your tweaks add to this?

[chris-morgan]

I did it this less efficient way because I had forgotten about visibility: collapse.

Still, my version does save one more pixel (this is compatible with the superior visibility: collapse approach):

#nav-bar {
  /* Let’s save another pixel. */
  border-top: 0 !important;
}