Vertigo Tabs per Erwan Ameil

Goal of this extension:
- respect the user's system theme (colors, font sizes)
- should just work => sane defaults. If possible, no configuration should be necessary.

*Please be aware that drag-and-dropping files/urls on the sidebar is not yet supported.*

* WebExtensions cannot automatically hide the horizontal tab bar for now. *

The current workaround is to add `#TabsToolbar {visibility: collapse;}` (without quotes) to the `[your_ffox_profile_dir]/chrome/userChrome.css` file (create it and the `chrome` folder if necessary).