A new project has been started by Mozilla with the aim of splitting up Firefox over several processes, with one process for the user interface, and one process for each open tab.
This is similar to both Google Chrome, and Internet Explorer 8, which both use multiple processes. The advantage of this is that if one tab crashes, it only closes one tab, rather than all open tabs, increasing the general stability of the browser.
The multi-process approach also allows the browser to take use of multiple CPU cores which are increasingly becoming the standard.
At this stage, it doesn’t seem like we will see a full release of this technology in Firefox for at least a year. More information on the project can be found in the Mozilla Links blog.