I have never used Arc, so don't know this feature. But you could pin tabs now, which could be a replacement of your use case.
Also with * you could search through BM in your a search bar.

On top, For Zen vs FF, if you want to play DRM protected content, that's sadly not possible in Zen, but in FF.

With the last update of "plain" firefox I'd also give it a try before using zen. Firefox supports vertical sidebar and tab groups.

I'm using immich.app hosted via pikapods.com and am really happy so far. The Immich app is really good. You can easily share photos with your friends. It has good face recognition and other AI features

Include all available apis inside of the open API spec openlibrary.org/swagger/docs