The overhead is minimal since we're not doing process-per-tab: we cap the number of processes to 4 by default. So if you have 500 tabs, loaded or otherwise, you're not going to have 500 processes. I believe we spin up the processes on demand, so if you have 499 unloaded tabs and 1 loaded tab, you should only have one content process, but I'd have to source dive to confirm that.
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.
Four is a reasonable default, especially given that multi-process is so new. It's possible that the default number may increase in the future, but that will depend heavily on memory usage.
Not to do process-per-tab and 500 tabs. Each browser process is several tens of megabytes, somewhat unavoidably (that's assuming you do some sort of embryonic startup and then forking so const non-relocatable data can be shared; if you don't it's a lot more). So on pretty much any consumer hardware you would be constantly swapping at best.
Which is why there is not a single browser that does process-per-tab. Chrome certainly doesn't: once you have more than a few tabs they start multiplexing them across processes.
I hope that you ask them to add it as an option instead of replacing the current model. Firefox already uses my 8GB memory quite easily when I have many tabs open.
They are also working on a UI for changing that max process count (don't think that's in Firefox 54, but I could be wrong), but that UI currently only offers values 1 through 7, as it's just really unlikely that a normal user would actually want to sacrifice their RAM like that.