The one feature I wish for is for tabs to be in the side bar instead of across the top. There are several reasons for this.
• On a typical laptop you can fit about 9 tabs on top, but possible side tabs are more like 28 depending on your type size.
• Computer screens are predominantly wider than tall, so vertical space is at premium. Tabs at the side frees up two or 3 more lines of vertical space, which makes for less scrolling.
• With more than 6 or 7 tabs on top it is impossible to read the full titles of the tabs, but with side tabs all tab names can be read.
• On tabs across the top it is hard to have a good way to see which tabs were opened from which source tab. With side tabs you can arrange it like an outline with indents showing subsidiary tabs.
• With the tab bar crowded on top there is no space for a close button/x or other controls unless you swap out the tab icon and tab name (making an unintuitive invisible interactive element) on the side tab there is room to spare.
Pretty much all these features can be seen on the extension Tree Style Tabs for FireFox