You have to keep in mind that a large fraction of Firefox users are still using 32-bit builds on Windows. For those users, the memory a process can use is capp

You have to keep in mind that a large fraction of Firefox users are still using 32-bit builds on Windows. 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.