> Tree style tabs enables vertical tabs It's not just vertical tabs (which MS Edge has an option for), it's tree-structured vertical tabs. If you open a link i

> Tree style tabs enables vertical tabs It's not just vertical tabs (which MS Edge has an option for), it's tree-structured vertical tabs. If you open a link in a new tab, the new tab will become a child of the original tab, making it easy to identify related groups of tabs. The subtrees can be expanded/collapsed. There are a lot of configurable options (e.g., to automatically collapse a subtree when you open a new one). More info on the extension's page: https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/ https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta... As someone pointed out in a sibling comment, Sidebery gives you a similar UI. I haven't used it as much, but it seems to work OK too.