For one thing, Firefox 64bit support is not halfhearted at all for Linux and Mac, but fully and officially supported (Windows is a different story).
For the other, split processes are still in the plans but postponed indefinitely while other ways of making the UI more responsive are being prioritized - Mozilla simply doesn't hve the manpower to do everything at once. The problem also isn't just add-ons but also that a lot of current Firefox UI has to be rewritten to communicate / react to website content asynchronously and indirectly, which it doesn't do right now.
Basically, it all comes down to being a lot of work, and a ton of QA and the manpower needed for this currently is being spent on things that are deemed higher priority like making the UI snappy to react in other ways. Volunteers doing work on achieving split processes sooner are of course welcome. It's not as simple as a switch though, there's a lot of work to be done before such a switch would leave even the current Firefox UI reasonably working.
Using split processes is much easier for something rewritten from scratch like the XUL (tablet) UI for mobile devices or - what this article is about actually - Boot to Gecko.