Firefox does both of those things out of the box with no extensions.

Yep, but admittedly the vertical tab UX is not the greatest. You either have them always be visible with an option to toggle by clicking the sidebar icon (no keyboard shortcut option afaik), or minimised as icons that expand on hover with an awfully annoying animation.

Looking at Zen, I really don't understand how Mozilla fail to capitalise on their browser, and build up a similar experimental project based on Firefox like it. It seems that many of these small QoL improvements could make a big difference. They have such a huge budget, and they waste it on inane things. Their fancy search deal with Google has made them complacent, and neglect one of the few things that ever had any real worth. Curious to see how it develops with the recent Google ruling. And to be fair, it does seem like Firefox development has picked up a bit lately—maybe even due to Zen's competition, who knows.

Vimperator plugin used to do the rest, but maybe that is no longer needed or working.