Gregory's Server
Haven’t heard of, but doesn’t seem advantageous as I’ve been using MultiFirefox since ... decades?
(A front-end to Firefox & related browsers’ profile managers allowing multiple simultaneous instances & versions.)
This gives me one instance for each site that requires & is worth either JS or cookies (not many), one for general use which deletes everything at close (close early, close often!), and 1+ for local/development use. Extensions are also isolated – mostly a plus I think.