I realized the numbering scheme last weekend when I downloaded the OS/2 release. I went to unzip it and noticed the last one I'd downloaded was the 1.3b version.

I don't see an "official version scheme" but it can be deduced by looking at the [link|http://www.mozilla.org/releases/|releases]. I see the sequence has been:
1.1a
1.1b
1.1
1.2a
1.2b
1.2
1.2.1
1.3a
1.3b
1.3

I'd prefer they used 1.1alpha, 1.1beta but suspect they were concerned about scaring off users. So now, instead of scaring them off. they just have confused users :-)