I've switched to Zen browser recently (I like it's spaces and folder structure to organize work on a daily basis, same as Arc). fFox nightly is not there yet but it's getting close with tabs on the left..
On top of that, Zen can be personalized with CSS. As someone who spends a lot of time in the browser, it's been awesome to be able to tailor it to my needs. https://docs.zen-browser.app/guides/live-editing
Arc is still irreplaceable for its true separation of tab and window, it’s like tmux for browser, I haven’t seen any other browser do that.
In Zen, if you open two windows, they have different set of tab instances; in Arc, you have same set of tab instances.
If you can raise a github issue, we can get to work asap https://github.com/browseros-ai/BrowserOS