I still use Arc as my primary browser. I prefer tabs on the side - I like how you can just drag tabs up and they will be saved (like Bookmarks, but I actually use it) I like pinned tabs which I use for calendar and my other top used "web apps"
It also has good support for profiles and spaces. For example, I have a "Work" space, a "Demo" space (with tabs open for sales demos), a "Personal" space, and even a "Travel" space for travel planning stuff.
And another killer feature is the ability to "route" specific urls to specific spaces, so for example I can have all github links open in my "Work" space.
It's a great browser, and I hope Atlassian doesn't ditch ongoing support for it.
For what it's worth, and from what you've described (I haven't used Arc myself), most of those features are also available in Firefox with the Sidebery extension. Instead of "spaces" it has "tab panels", with a horizontal row of icons above your tabs that lets you switch to different panels of vertical tabs. You can pin tabs in a panel, you can setup URL patterns to automatically move tabs to the right panel, and it works with Firefox's multi-account containers so you can even have an URL automatically re-opened in a specific container associated with that panel.
[Side note: I'm hooked on Firefox's multi-account container feature because I can have different containers for general use, for work, isolated social media containers, etc, without needing an entirely different profile as in Chrome/Chromium and its variants. I've tried Vivaldi and other Chrome-based alternatives recently, but profiles are just too big of a separation by comparison, with separate extensions, bookmarks, settings, etc. I want all those things in one synced account where I can just open new tabs with their own set of signed-in accounts. Does Arc's profile feature have the same level of separation as Chrome? Am I missing something about how Chrome profiles work?]
And for anyone concerned about Firefox's recent statement about personal data, there's a great Firefox-based alternative called Waterfox that adds some nice features and has a much stronger emphasis on privacy.
Yes, most of these features are available in Brave. You have Vertical tabs, split view, pinned tabs, grouped tabs, tab sneak previews, tab search, in built AI assistant. Instead of 'spaces' you have old fashioned profiles, single windowed space switcher would be nice to have in Brave. Arc also has a nice Tidy tabs feature.
Zen is a Firefox-based browser that has copied a lot of Arc's good ideas, if you are looking for an alternative.