According to recent comments (continued on the next day's thread), the win32 compiler that Mozilla use is approaching the 4GB limit, after which LibXUL (which Firefox depends upon) will no longer compile.
It's currently at 3.5GB, and at the current rate, will reach the limit in approximately 6 months: Chart of memory usage of LibXUL during last 90 days
While I think that Servo will produce a more decentralised design than Gecko and XUL, the memory limit will be reached well before that. With Windows XP support ending next year, Mozilla should consider migrating to x64 as soon as reasonably possible, keeping x32, but focusing on stripping large and extraneous code above new features.
Real Users are afraid they'll break the machine -- but they're never afraid to break your face.