I would like to see vertical tabs baked in. MS Edge now has it out of the box!
I currently use addons for this - but there's no way to disable the top tab bar, which duplicates the tabs and takes up space.
Vertical tabs (with top bar disabled) would free up more space for web content, which is (usually) vertical. Most screens are widescreen (RIP ThinkPad 4:3 ratios), so the less browser UI at the top/bottom, the more content we can view.
The original post received 154 votes and as such was the 3rd highest voted post in the "General" category.