> The fact that FF runs in a single process seems to be a major security issue since the web views are not sandboxed.
Running content in multiple processes, by itself, doesn't help security at all. It does help stability, since if one process crashes the rest of the browser need not; that's particularly important if running plugins like Flash, which crash in a stiff breeze. But if untrusted content manages to exploit a separate browser process that doesn't have any additional sandboxing applied, there's nothing standing between it and the rest of your system.
Additional security, however, comes from then applying sandboxing features to those separate processes, following the principle of least privilege.