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New Firefox Project Could Mean Multi-Processor Support
suraj.sun writes with this excerpt from Mozilla Links "Mozilla has started a new project to make Firefox split in several processes at a time: one running the main user interface (chrome), and another or several others running the web content in each tab. Like Chrome or Internet Explorer 8 which have implemented this behavior to some degree, the main benefit would be the increase of stability: a single tab crash would not take down the whole session with it, as well as performance improvements in multiprocessor systems that are progressively becoming the norm. The project, which lacks a catchy name like other Mozilla projects (like TaskFox, Ubiquity, or Chocolate Factory) is coordinated by long time Mozillian, Benjamin Smedberg; and also integrated by Joe Drew, Jason Duell, Ben Turner, and Boris Zbarsky in the core team. 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."
I think you are a bit confused.
With linux, the only difference between context-switching between threads and between processes is the update of the page tables and the flushing of the TLB. Not normally a big deal.
Also, I'm not sure where you get the idea that interthread communication happens in userland--threads share memory, file descriptors, signal handlers, etc., but things like sockets/pipes need to go through the kernel. Processes can be made to share memory too, it's just a bit more work to set up, and you need to be explicit as to exactly what is being shared. (Which can be an advantage.)
Perhaps you're thinking about synchronization primitives which do not require a syscall in the uncontended case--if so, those are valid to use between processes as well.
Multithreaded apps have the potential to be faster than multi-process ones due to the lack of TLB flush, but they're more fragile due to the shared memory. For something like a browser which is often prone to crashing on crappy plugins, it makes sense to aim for reliability.