Firefox tries to minimize breaking API changes, but there have certainly been plenty of them. While avoiding API changes presumably made the switch to multiprocessing more difficult, it didn't stop it; they made the switch. The fundamental problem with extensions and multiprocessing (at least from the perception of someone who wrote FF extensions) is the obvious one: Rather than having everything for all windows in a single thread, now you have them in different processes; communication goes from literally nothing to IPC.