A look at an upcoming feature in Waterfox -- the sidebar!
Waterfox is integrating Tree-Style Tabs as a vertical tabs solution, complete with image tab previews and optimizations for tabs outside the scrolling.
First time I'm hearing about Waterfox, but it's strange they're leaving in the regular tabs. I hope they add the option to remove them instead of having to use custom CSS like in FF.
It would minimise until I move the curser to the left, which then it would pop out. If the curser leaves the tab area it would minimise itself. It used to be a function once upon a time before Firefox changed how extensions work.
I couldn't get used to pressing a hotkey/button to hide/unhide it when the current version came out. I did find some custom scripts that might work from some casual searching though.