It's simple. There is no way Firefox can guarantee anywhere near the level of stability and security Chrome offers without a process per tab. OS primitives operate in terms of processes (scheduling, memory, sandboxing, and so on) and Firefox will not be able to use any of them. I really want to use Firefox, but I'm almost certain not doing a process-per-tab will be the last nail in its coffin. The code will be more complex to maintain, and no advantages in security or performance will be gained, leading to less users and thus less maintainers.

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.