[cosmic_cheese]

I tried Sidebery for a while among other vertical tab extensions and found them all too flaky and tacked-on-feeling for my taste. Needing to edit userCSS to hide the redundant tab bar kind of sucked too. Now I don't use vertical tabs unless the browser has them built in as a first-class feature.

[yencabulator]

For what it's worth, Firefox now has vertical tabs as a first-class feature. (FF does groups but not trees, if you were a Sidebery power user.)