At oldapps.com where previous versions of Firefox are available, I see versions that are familiar (i.e. Firefox 11.0, 12.0, etc.) but I also see Firefox 3.6.26 and 3.6.28.
The file size of the 3.x.xx versions is about half that of the 10.0, etc. ones. The latest 3.x.xx update was just 4 months ago so it certainly isn't ancient.
What does the version with the larger file size do that the smaller doesn't. When I look at the supported systems detail on each, I frequently don't see a difference.
This question just falls into the "curious" category.