The news that Firefox is to get multiprocess support is a perfect example of how we all benefit from strong competition in market segments.
The project, called Electrolysis, will see Firefox use separate processes for different aspects of the browser such as the UI, the pages loaded into tabs, and plugins. There are huge benefits to taking this approach. It improves performance, especially on multi-core systems, the UI should be more responsive, and switching to multiprocesses should mean improved stability. Mozilla is also eyeing the potential for running web content in protected or isolated mode, thus protecting the system from malware.
Currently Mozilla are working to get the basic code working right and handling plugins, after which comes the fixes. And there will be a lot of these needed - anyone who thinks it's easy to update legacy code in this way hasn't ever had to do it!
This switch to multiprocess makes a lot of sense, given that dual-core processors are now the norm and that quad-core is growing in popularity daily. Add to that that Intel's Core i7 can run two threads per core and it's easy to see how multiprocess is the future for all but the simplest of apps. Why spend money on all those cores if you aren't getting the benefit?
Note: At this point I'm assuming that Mozilla will implement multiprocess as an option and allow users to enable/disable the feature. This would prevent millions of Firefox users crying out in terror as their add-ons stop working!
Side thought: I'm pretty sure that the reason Firefox is getting multiprocess support is because IE8 has it!