He's using something tab-like, the task bar or whatever it's called on Mac. I use my (linux) task bar to see my separate apps (like everyone does), with sub-"apps" in Firefox tabs. His "tabs" are just migrated up to his task bar as separate instances of Safari. Same thing, different implementation. They both bookmark locations of concentration. The difference is that, on the Mac, for jwz, it works well with Safari and not with Firefox.