> I am not sure if someone like Opera was earlier

OmniWeb 5.0, 2004: https://arstechnica.com/gadgets/2004/02/ow5/3/#tabs

Yep, OmniWeb was the first place I saw the of the vertical tabs back in the mid-00s. Blew my mind.

The concept of “drawers” that OS X had at the time was well suited for this kind of thing, with some “free” (in terms of dev effort) flexibility of allowing the user to choose which edge they unfurled from. It’s too bad they fell out of style in favor of much more static sidebars.

Opera 4 had them in 2000. Opera 3 might have, but it won't run on Windows 10 so I can't easily check.

edit: Started up a VM -- Opera 3 didnt have tabs at all, although it did already have an MDI.

I used Opera over other browsers for a long time for performance reasons, it had M2, my favorite email client ever to this day, which got rid of traditional "folders", there's an alternate timeline somewhere where Opera won the browser wars and they already have flying cars and a federated web.

Windows common controls had side-aligned tabs around 1997

OS/2 2.1: https://guidebookgallery.org/screenshots/os221#general, released December 1993.