OmniWeb (from Omnigroup, developers of OmniGraffle, OmniPlan, OmniOutliner et al) was their web browser offering in a particularly fecund period of browser development in the early 2000s, which had vertical tabs. The tabs themselves were little thumbnails of the contents of the web page in question.
I used OmniWeb as my main browser for many years, right up until Firebug and Firefox became the preferred development environment. Great browser, a lot of great UI ideas out of that shop.