This is only a bugfix release, fixing a few of the glaring problems that crept into the 1.3 release, so it would normally be released when its ready.
The more testing people do on these cadidate builds (and the less problems they find) the quicker 1.3.1 will be officially released (at least this was the case with 1.2.1 which was created for a similar circumstance after DHTML problems with 1.2).
So far, so good.
This will be the second-best Mozilla release ever, after 1.0.
I'm not sure whether 1.4 will be better than 1.0 or not, but it's close, thanks to a lot of the bookmarks stuff being fixed.