Let's stick with the original subject, because the rapid update applies only to Firefox and Thunderbird, and not Seamonkey. As far as I recall, SM 2.1 was based on FF3.6.x or 4.x, while 2.2 parallels FF5.x. As for the mail part, I don't know nor care. I know that is, immensely, easier to deal with Seamonkey mail than Thunderbird 3.x and later.
As for the issue of the Toolbar, and addons, favicons, and whatever, I keep seeing the same complaints which I saw when Netscape 6/7 came out, along with the Mozilla browsers, firefox, and whatever.
It has been known for several years that working with one profile, and two or more programs is risky, if not disastrous. As far as SM2.0.x, and 2.1, they should be installed in different folders, and each one should have a separate profile. I have been doing stuff like that for years, with no problem. The only times I had difficulties was when I let the programs do their thing, rather than controlling the situation. SM2.0.x and 2.1+ profiles are located in different folders, and their design is different, so it stands to reason that they could not be shared.
As for favicons, the site works or not. The favicon is a signature, an imprint, almost an ad for the site. Should I, even, look at a favicon.
Of course, the addon incompatibility is a well known issue. Addons are called addons, because they are added to the program, almost always from outside, and it is up to the individual to make them compatible with the program. More often than not the addon developer is unaware of the issues, or, in many cases, he/she could care less to spend any time. I have been pretty successful using incompatible addons, with Nightly Tester Tools, and addon which almost constantly updated.
I like Seamonkey, and I have, especially, like its improvements after v.2.0.x. Considering the fact that I have been using separate profiles, even when testing a or b version of the future program, as Nightlies or installer versions, I have not had problems. At this point I am using two incompatible version with my SM2.2.
As for 2.0.14, it has gone long ago, along with its Netscape derived profile.
I will, no longer, comment on difficulties encountered by others. I believe to go with the flow, not against the flow. And, if the current is too swift, I stay out of the water. All I need to do is to drive a few miles east, and watch the mighty Mo( Missouri, nor Mozilla) in action.
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:5.0.1) Gecko/20100101 Firefox/5.0.1