It's a matter of priorities. 64-bit is important, but for almost all users, it isn't an immediate concern to use 32-bit on windows, nor would they get a big benefit to move to 64-bit bit there.
There are other things that the devs are focused on, that have more tangible immediate benefits. That's all it comes down to. At some point though I would imagine 64-bit would become higher priority.
edit: Regarding multiprocess, it is already used in FirefoxOS, for example. Enabling it on desktop would be hard because of all the existing addons, on mobile though it has been used for a while (the Android browser was multiprocess, now it is multithreaded, and the FirefoxOS browser is as I said multiprocess). Since most of the growth in the browser market today is in mobile (sales of desktop PCs actually declined this year), that is a forward-looking approach I would say.