Firefox because it's plugins make mobile browsing bareable. I have a folding phone so I wish that Firefox would hurry up and copy Chrome's multi window feature, and then it would be the perfect browser for me.
I use Firefox on desktop too, but I've been experimenting with Arc the last few weeks to see what the fuss is about.
There's also Zen browser that's Arc-like and based on Firefox instead of Chromium. Zen lets you sync tabs with Firefox elsewhere (including mobile Firefox), run the full uBlock Origin, and it is a fully open source browser.