The Mozilla "Electrolysis" work on making a multi-process Firefox and Gecko engine is working out in Firefox OS, but there hasn't been much work in recent time on the Firefox desktop browser to better utilize multiple threads. Code is still happening, but it isn't yet up to par.
If using a Firefox nightly build and enabling a Firefox preference setting it's possible to ruin in a multi-process mode, but it's not entirely complete. The work basically leaves one process for the UI and one process for all of the content -- regardless of the number of tabs. All "basic browsing functionality" works under the multi-process mode but features like printing and developer tools are not yet working.
It's not yet known when the multi-process Firefox will ship by default, but there's many more technical details in a lengthy blog post by one of the developers, Bill McCloskey.