Comment by polyfractal

Tree Style Tab is another option, allowing vertical tabs with nested hierarchy: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

As someone that likes to use open tabs as a sort of todo list, I can't live without that extension. Allows me to keep different trees open for various purposes: PRs in one tree, Issues in another, academic/scientific paper to-read list, forum posts, etc