Mozilla's multi-processing project is called Electrolysis[1], and the process-per-tab variant is used in mobile versions of Firefox. I think it's pretty far off for the desktop version, and as long as blue sky ideas go, I'd prefer to get the security advantages without the increased memory consumption (maybe with shared heaps within a privilege level, whether or not multiple processes are used).
As far as increased responsiveness, that's addressed by the Snappy effort, which you can follow here[2] as a sort of sister-blog to Nicholas' MemShrink reports.