Personally I don't find tree-style tabs appealing at all because (1) I usually have few pages open on the same domain, but I have many pages open on different domains. And (2) I don't want a vertical column of content width lost to a sidebar.
Limiting tabs to a single row per window is awful for usability.
You have to understand that this is a pretty unusual concern. Most of the world struggles with screens with aspect ratios optimized for watching TV, making vertical space very precious but leaving plenty of excess horizontal space that cannot be effectively used by the majority of web pages that only use a single column of text.
You are right about that. I use two external monitors, one of which is rotated 90°. Guess which one I keep my browser window on? :)
- Multiple-row tab bar.
Such an obvious improvement over tabs scrolling off the screen sides or squashing them like Chrome does, I don't know why it's not being considered.
It's already been done by Tab Mix Plus (or was it Classic Theme Restorer?). They only need to copy it and polish it a little.