I, no longer, have the uninstall.log file, since I removed TB2.x, but it is possible that something remained in the Thunderbird Program File folder, since that's where my TB2.x was located, until I uninstalled it. That's where TB3.x went by default. Initially, I hated TB3.x, and there are a number of reasons why, I, still don't like it. So, I downloaded TB2.0.0.25pre, a nightly, and specifically, placed it in a folder of my choice. By default, it was trying to install in the same folder as 3.x. The shortcut was named(not by me) Mozilla Messaging for 3.x, although it was thunderbird.exe in the installation folder. The shortcut for the nightly was also called thunderbird.exe, and I renamed it (the shortcut, not the originally file) to avoid confusion. That nightly hung around for months, and coexisted with 3.x, from the earliest version to 3.1.6. During this time, 2.0.0.25 was my default mail program. Finally, I decided, first, to make 3.1.6 default, and then uninstalled 2.0.0.25pre. And, that's were confusion ensued.
As for Firefox, I had placed a rhetorical question, as I had to create a new profile to run the pretty stable 4.0b7. At another forum, I was told that the disappearance of the addons in b7 was due to a corrupted profile.
But since b7 is the first Status Bar-less version, I thought that I should look at b8, which is available, only, as test version. I did not bother looking if and where it differed from b7, but decided to check how it would do with the addons which were disappearing with b7 , by using the same profile, formerly, used by b7. And, so far, I have not lost any addons. So much for corrupted profile.
At this point, I am waiting for a stable b8, or better, a stable 4.0 period, so that will do away with one Firefox.
And, 3.5.16 is another nightly, which I keep, just in case, but I can let go of it, any time. And, having 3.6.12 portable is redundant, but good to have, in case something happens to my Thumb Drive portable.
There has, never, been any confusion before 4.0b7. If anything, it should go.
And, for the record, I have, always kept an older version, till I was satisfied with the present one, and tested a newer version, and, not once, have had any issue.
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:2.0b8pre) Gecko/20101113 Firefox/4.0b8pre