Reason? As I had said before, FF5.x would not open if I had unchecked the box next to "do not ask on startup", in spite of the fact that I had modified the Target line, by adding -p profilename. The program would start with the profile manager. This is not so bad, but if I placed the check mark, with the profile highlighted, the default 3.6.16 would open with the FF5.x profile. If I placed the check mark next to the 3.6.16 profile, but then used the 5.x profile (still unchecked), a check mark appeared, next to the 5.x profile, so that opening 3.6.16 from a link would open it with the 5.x profile.
In other words, everything which I had been taught since profiles became available, years ago, is defeated by the propensity of 5.x to attempt to use the wrong profile. I, simply, cannot operate this way. I reported the issue, but received no reply. Under these conditions, I cannot consider testing 5.x. 4.0 does not do this, although it tried, initially, if I had left the check mark next to its profile, to open 3.6.16 with the 4.0 profile.
I will, probably, move to 4.0 as default, but, if the issue persists, I will have to give up on Firefox. At this point, I hope that it is, just, a case of a knuckle head developer making some dangerous change.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:184.108.40.206) Gecko/20110319 Firefox/3.6.16