As well as allowing the software to take advantage of multi-core microprocessors to boost responsiveness, the enhancement would also improve browser stability, the company said in a news blog on the subject.
The unnamed project will happen in several phases, with the phase one 'bootstrap' due to complete by the middle of this July. Further phases will add and debug more complex elements of the new browser, with phase two due to complete by November.
No timescale is given for the phases beyond that point in time, which suggests that multi-threading Firefox could turn out to be a project that takes well in 2010 to even near completion.
The goal is to end up with a browser capable of running each tab as an independent process, which would stop a hang in one from crashing the whole program, as would currently be the case. "Security sandboxing will be covered in a later phase," notes the blog.
Other uncertainties include whether Mozilla will start using the open source networking stack taken from Chromium - also used by - in place of Mozilla's own Necko.