I disagree with this decision by Arc, and had the same reaction you did, but I just want to note that I'm very glad I gave it another shot months later – it really is a fantastic browser, much better than FF or Chrome.
I feel like the only actually cool feature of it is the way tabs are organized, but you can get the same thing (even better, actually) by using the Sidebery extension in Firefox. The only difference is that it doesn't look as pretty by default, but both Sidebery and Firefox support custom CSS so you can make it look like Arc if you really need to.
As someone who hasn't used Arc, can you explain how it's better than FF?