There has been one case, with loss of default, when another a similar program, like a mozilla was present. The loss of mail default has been less common than the loss of default browser.
But, on the other hand, if the practice used by many people to use the same profile, and possibly the wrong sequence of uninstalling of the older version, is followed. 4.x is no problem, but anything else can be, and it gets worse if the new program is installed on top of the other.
It is beyond me why the programmers have not figured this out. Installation should not be allowed to proceed if there is any conflict.
I, always, create a new profile for a new version, but there is something strange about recycling profiles. The other day, I downloaded a 1.8a4 nightly, working out of the unzipped file. I decided to use a former NS7.02 profile. Everything worked fine, but when I went back to 7.2, I had lost my mail default, not the browser default.
If you created a profile, just, for NS7.2, and you did a proper uninstalling of 7.1, and don't mess with mozillas, I have no answer.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; rv:1.7.3) Gecko/20040911 Firefox/0.10