My post was pretty clear, I thought, and the point is summarized in one sentence:
The moral of the story is that if someone wants both version he/she should rename the Mozilla Firefox shortcut.
I have had two versions in stalled in the past, several time, with no problem, but the name of the shortcut had been different. It seems that Mozilla Firefox is the default name of the shortcut, and if there is one shortcut of that name, the installation of a new version will overwrite The Target line, and the Start in. Had I renamed the shortcut, there would not been any issue.
In other words, there is no reason to have fears in having any number of version at the same time, provided the second, and later versions are Custom installed, and separate profiles are used. Why should there be any corruption?
Technically, I could have as many versions as there had ever been (hard drive space permitting).
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:13.0) Gecko/20120615 Firefox/13.0.1 SeaMonkey/2.10.1