Mozilla has added much-requested vertical tab support to Firefox’s nightly builds, making tab management easier for users.
The feature can be enabled in the latest test version by adjusting sidebar settings. After a quick restart, tabs appear vertically in the sidebar for improved workflow. Watch for more sidebar updates coming soon!
https://connect.mozilla.org/t5/discussions/here-s-what-we-re-working-on-in-firefox/td-p/57694
