After using FF for the past 7 years, every day at work I switched to Arc. It's been __okay__. I'm not a fan of them trying to nix the bookmark system and much prefer FF's implementation of just good old fashioned, indexable bookmarks. If FF can manage a good implementation of vertical tabs, I'd switch in a heartbeat.
I love that I can start a web search open a bunch of links, each in a new tab, then once I have what I need, I can close the parent tab and all the child tabs are closed automatically.