Mozilla has set out to incorporate a feature in Firefox which will allow it to run divided among numerous processes. The plan is to have the browser's user interface pegged to a parent process, with the subsequent child processes spawned on-demand per tab. Implementing this structure is expected to boost the browser's overall stability, as well as its responsiveness - especially for those running multi-core CPUs.
By separating processes for each content instance, if one tab were to crash, the entire session would not be compromised. Additionally, security can be enhanced by stripping content processes of their rights, allowing them to compute but not read from or write to sensitive areas - everything would be proxied through the parent UI process. If this "sandbox" approach sounds familiar, it's probably because both Google Chrome and Internet Explorer 8 utilize the behavior to a degree.
The development team is being coordinated by Benjamin Smedberg, and the initial tier of the four-plus phase creation is scheduled to be completed July 15 of this year. I wouldn't expect to see a polished release until next year, though. Phase two has a scheduled goal of November 1 and the following steps currently lack ETAs.