> IMO the real solution, given that we overcommit, is for firefox to check that % RAM usage

Doesn't it to that already? I remember 3-4 years ago I use to watch in amazement how people complained that Firefox ate over 2GBs of memory, whereas I ran it on a laptop with 2GB total, and FF rarely hit 1G even with dozens of tabs open.

EDIT: Mozillazine seems to confirm my experience: "These numbers will vary because Firefox is configured by default to use more memory on systems that have more memory available and less on systems with less" (http://kb.mozillazine.org/Reducing_memory_usage_-_Firefox)