[FR] Vertical toolbar (Brave sidebar alike)

[mr-cheffy]

What feature would you like to see?

Brave browser features a sidebar that's placed at the opposite side of the vertical tabs (although it can be configured to be next to them, on the same side).

It could hold:

• Clickable Toggles for Firefox/Zen sidebar sections, which could, in the future, include the upcoming AI Services
  
• Docking space for toolbar's items that can be manipulated through the "Customize toolbar" view. Including shortcuts for navigation, extensions, devtools and screenshots.
• ???

Other details

No response