At the end of the day, you're still aiming to load, in FF 66, just as many tabs as in FF pre-66. FF's total memory usage should be about the same whether you're using 4 processes or 8. Sure, if each FF process now takes care of fewer tabs, then when OOM does happen, the FF processes have a lower OOM score and are less likely to get killed. But something will get killed regardless, just maybe not FF. That's like trying to avoid punishment after a prison brawl by keeping your head low: someone will get punished regardless, just maybe not you.
For those users, the memory a process can use is capped at 2-4GB (depending on whether the OS itself is 32-bit or 64-bit and a few other things).
The most common OOM crashes on Windows are running out of virtual address space, not running out of actual physical RAM.
In that context, having more processes in fact gives you more address space and reduces the chance that you will run out.
It makes sense then.
In practice some of that is reserved for the kernel, so you get less for use by the process itself. Historically 2GB on Windows, though there were some non-default compilation/linking options you could set to get 3GB.
A 32-bit process running on a 64-bit kernel can get 4GB of address space.
And yes, lots of computers have >4GB physical RAM, even if you don't count swap/page files.