* Remove the #titlebar element, and ensure toolbars that comprise the titlebar get a .browser-titlebar class
* Ensure we call TabBarVisibility.update when initializing with verticalTab=true
* Adjust TabBarVisibility logic so we allow for the vertical tabs case
* Give #navigator-toolbar z-index: var(--browser-area-z-index-toolbox) to ensure the urlbar view doesnt get clipped behind the lower elements
Differential Revision: https://phabricator.services.mozilla.com/D222059