Multiple threads are supposed to make Firefox more stable and faster. The feature is said to be integrated until late 2009.
Firefox gets multi-processor support
The Mozilla Foundation wants to upgrade Firefox with multi-processor support. The Open Source web browser is supposed to run its tasks on several cores simultaneously. Accordingly Firefox will be split up into the user interface (first thread) and the web content (thread per tab possibly). The competitors are using this technology already: Internet Explorer 8 and Google Chrome are working with multiple threads. But the biggest advantage for the browser is not the speed benefit but the higher stability. IF one tab crashes, the remaining tabs and the web browser itself aren't affected.
Performance of course benefits from multiple threads, too, although the advantage is not as big as known from other applications since the limiting element of a web browser is the internet connection or the server at the other end of the line.
The integration of two threads, which separate the interface from the web content, is scheduled for mid July. Phase two with the interaction of the two threads is planned for November.