one process per tab is specifically why I quit using chrome, it wasn't working even for small amount of tabs (100) it ate RAM like crazy. Mozillas approach is better, user can control how many processes are created - users have control.
If there was one thing that caught my attention with Chrome back when it was released (2008?) it was its reliance on OS primitives (processes) as the building blocks for a stable and secure browser. This is essentially the same argument the Varnish folks did when comparing to other proxy solutions like Squid back in the day. I don't understand why Firefox is taking this route.
It has no security benefit without Site Isolation (which isn't unconditionally a process per origin either for performance reasons). In both Chrome and Firefox, any site can embed another cross-origin site in an iframe, and it will share a process (and a main thread).
Chrome does not use an unconditional process per tab. Nobody would.