Chris Jones just posted a Multi-process Firefox, coming to an Internets near you write-up to his blog, about the Electrolysis project (coordinated by Benjamin Smedberg, with Joe Drew, Jason Duell, Chris himself, Ben Turner, and Boris Zbarsky) that will add multi-process (one tab per process) support to Gecko. Chris’s blog posting has a video demo that’s worth watching, and last week Ben Smedberg posted a Electrolysis: Making Mozilla Faster and More Stable Using Multiple Processes write-up to his blog that’s also worth reading.
For details on status on the ongoing project, see the Content Processes page on the Mozilla Wiki, or go talk with the developers in real time on the #content channel on irc.mozilla.org.