When we started work on Chrome my favourite browser was a now-forgotten IE shell called iRider, which implemented tree-ish style tabs in a better way than anyone, and we did take multiple attempts at vertical tabs early on.

We didn't ship them because they were only a mitigation of the too-many-tabs problem, not a solution, and they didn't really fit our model of 'tabs are titlebars'. They were also never going to be default - most people did not have that many tabs, and we had a very strong opinion was that we shouldn't have configuration - it was better to very strongly execute on one vision we loved and risk losing people (but hope the quality would bring people along), than to execute and support multiple directions poorly.

The world has changed a lot since I last worked on Chrome ten years ago, so as an outsider I'm excited to see what the team currently attempting it can do.