[TranquilMarmot]

Firefox has vertical tabs and tab groups now; those aren't specific to Zen

[paldepind2]

Correct me if I'm wrong, but neither vertical tabs nor tab groups are fully ready and shipped.

From a quick search, it seems that you need to make edits in `about:config` to enable tab groups and use nightly to access vertical tabs.

[homebrewer]

Vertical tabs are available in the latest stable release. They're pretty basic compared to tree style tabs and sideberry, though.