> the ux of Mozilla vertical tabs experiment is weird and IMO bad ... the tab bar goes up on the side of the awesomebar and slides it back and forth as the tab panel expands and goes back.

Yeah this seems really whack. I like what Safari did: they have a left sidebar with several tabs (favorites, reading lists, shared links). Having these independent sliding spaces overloaded in the same area is clumsy.

The problem though is there is UI hierarchy between tabs and something in the sidebar like bookmarks. I can't think of a good way to place these relative to each other to impart that relationship, which is probably why they are independent for now. Hopefully they do something about that though.

Please share your UX problems on the Containers experiment's GitHub issues! Getting early user feedback is why we're testing more Firefox features as experiments. :)

https://github.com/mozilla/testpilot-containers/issues

> and slides it back and forth as the tab panel expands and goes back.

There's a "pin" button there that keeps it open. I know, this doesn't fix your problem, but I and everyone I've seen using it uses the pin button, and IMO it's a much better experience :) I've noticed that on most high-end laptops websites have way too much blank space on the side margins so pinning a useful tab bar in that space works fine.