Switching an application from single to multi-process is no small undertaking, especially with a project the size of Firefox. Despite Mozilla's best intentions to get "Electrolysis", the codename for its project to make its browser multi-process, into 43, the implementation has been pushed back to 2016 for version 46 at the earliest.
The tentative date for Electrolysis support was 15 December, when the feature would be merged into the stable release for Firefox 43.
The [Electrolysis] team estimates [Electrolysis] with a single content process will be enabled in Firefox Release by the middle of 2016.
If you hit up the browser's rapid release calendar, you'll see mid-2016 coincides with 46 to 48 — for the sake of reference, the current stable Firefox version is 42.
Given the complexity of moving to multi-process, it's understandable the Mozilla would want to push out the release. It's not something easily integrated piece-by-piece — at best you can slowly move certain browser functions to other processes (image loading, for example), but even then it can be a can of worms depending on how modular said function is, relative to the core systems.
On the bright side, the delay may give add-on developers some breathing room for getting their extensions ready for the switch.