Wikipedia claims the session restore features was added with 2006's 2.0 release. That's what I would have guessed, since that release was (famously) a giant step backwards in performance, which was never really addressed except by waiting for hardware to improve.

I'm holding out hope that whoever's responsible for Firefox abandoning "good, light, cross-platform web browser" as its differentiator stays away from Servo, at least for a while. That's a niche that's unserved currently, AFAIK.