

I am a novice, so if someone else is going to give different advice, consider disregarding mine.
If you want precise collisions you could manually add 2D Areas and align them with your map. Otherwise if you are happy enough with an approximation you could add a tilemap (collision only, no sprites) and use a different tile for each region.
I would go for the first option, considering that you don’t have too many regions. You don’t need any external program, as far as I remember, you can create polygonal shapes directly in Godot.
I’ve started using vertical tabs in Firefox as soon as I got the notification. I never thought I would have liked them so much.
Why are you asking for decent vertical tabs? Are they inferior to some other browser you have in mind?