Mozilla is planning to follow Google Chrome's example by making Firefox split in several processes at a time, one process will run the main user interface while another or several others run each tab in a separate process. This should improve security, stability and also performance on multi-core systems. A loose roadmap has been unveiled, which hints it will take about a year until this project is fully implemented.
According to the loose roadmap published, a simple implementation that works with a single tab (not sessions support, no secure connections, either on Linux or Windows, probably not even based on Firefox) should be reached around mid-July. Phase II will deal with the interactions between the two process types (chrome and content), and is aimed for November.
Phase III will take on adapting the APIs for extensibility, accessibility, and performance. By this time (not even guesstimated), we could see a release to play with. Finally, Phase IV will extend the previous development to support several content processes at a time. Security sandbxing will be covered in a later phase.