As I mentioned earlier, those of us who had been on the nightly builds for Firefox 4 (including both 32-bit and 64-bit Windows) were updated to Firefox 4.2a1pre. I thought that was a rather odd numbering scheme as theoretically the next version was suppose to Firefox 5. After some research I have determined that 4.2a1pre is a continuation of the nightly builds formerly label as 4.0b13pre. The 4.2 is just a place holder number (much like 3.7, which eventually became 4.0 once in the beta builds) for what will likely be Firefox 5.
Now, no official word yet as to when Firefox 5 might be released although Mozilla is considering a new development process which would push out a general release in about 16-weeks.