Mozilla has announced a new project that allows the web browser to run several processes at a time - one running the main user interface and several to run the web content in each tab. The benefit of this is that if one tab happens to crash, it will not take down the entire web browser.

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.

So it seems we won't see a multiprocess Firefox for at least a year or so. However, some decisions like taking Chromium's networking stack to replace Necko, could accelerate the process. As you may know, Chromium is the open source version of Google's Chrome.