This Firefox add-on arranges tabs in a vertical rather than horizontal fashion. It is heavily inspired by and borrows ideas from the excellent Tree Style Tab add-on.
Download & Install: http://people.mozilla.org/~rcampbell/VerticalTabs.xpi
Goals:
Ability to arrange tabs in groups (instead of arbitrary trees) plus some group-related features (collapse group, tab previews, etc.)
Very few other features and hopefully little or no preference settings. Instead a good JavaScript API might be provided for other extensions (e.g. JetPack SDK based) to provide additional features.
Native look and feel.
Readable and maintainable code, taking latest Mozilla practices into account. No eval() hacks.
License: Mozilla Public License Version 1.1