I was maintaining such a setup for a while and didn't have such luck. Sometimes it'd fully break and sometimes things would shift around due to other CSS changing, but whatever the case it's more than I had the patience to babysit.

> Hmm what's Sidebery missing?

The extensions themselves aren't lacking, but they suffer from being extensions and not part of the browser proper, which makes for occasional weird interactions and glitches and the "seams" between extension and host becoming tangible from time to time. Things just work better when the browser "knows" about the tabs being vertical and having deeper organization than is possible with horizontal tab bars, or is even designed around vertical tabs being the only option (like Arc/Zen).