If you have not used a 2.x version of Mosaic before, you should first read Help On Version 2.0 for an overview of things that have changed since version 1.2.
Version 2.4 includes the following changes from version 2.3:
Never ever EVER let anyone rush you into a software release, no matter how serious they think the trouble is! That said, I now hope to not compound my difficulties by rushing out Mosaic 2.4. I apologize sincerely to the 2135 people who downloaded Mosaic 2.3 source and/or binaries from our ftp server. I left a rather serious flaw in that version. The names of the toggle button widgets appeared just to the right of all toggle buttons. This made all forms that used toggle buttons look really bad. That bug has been fixed, and to avoid confusion, I have made a new release Mosaic 2.4. If this page seems sparse on information on changes and bug fixes, it probably means you have never looked at the help on version 2.3 and you should do so now. Embarrassed and very sorry - Eric