This sounds like a chicken-and-egg problem somewhat specific to Mozilla. Their users depend on a large stable of independently developed plugins, which must be updated to support a 64-bit Windows browser. I can imagine that many of those plugin developers are much less motivated to do the requisite porting (or fix 64-bit bugs after they port) before Mozilla ships (or even commits to shipping) a 64-bit Windows browser. But when plugins used by 80%+ of your userbase are nonexistent or buggy, releasing a browser becomes difficult: Everyday users will assume it's buggy because they don't make the distinction between plugin authors and Mozilla.