Orion[1] is another recent MacOS browser that released before Arc in 2021 and has vertical tabs. Edge had vertical tabs as an experiment back in 2020[2] and is full feature now. Vivaldi has had vertical tabs since 2015[3]. It might not count since it's not built in, but Firefox has had vertical tab extensions since 2007[4].
[2] https://www.howtogeek.com/697986/how-to-enable-and-use-verti...
[3]https://www.maketecheasier.com/vertical-tabs-browsers/
[4]https://github.com/piroor/treestyletab/graphs/contributors
https://piro.sakura.ne.jp/xul/_treestyletab.html.en#history
I am not sure if someone like Opera was earlier, but Arc is likely copying this.
OmniWeb 5.0, 2004: https://arstechnica.com/gadgets/2004/02/ow5/3/#tabs
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.
edit: Started up a VM -- Opera 3 didnt have tabs at all, although it did already have an MDI.