if all the Mozilla Firefox lies would be like what you described, I would be delighted. The lies apply only if you allow auto-updates, or you don't read what the alleged update is about.
3.6.19 was supposed to apply, only, to MAC users. Since I am not a MAC user, I cannot comment on the usefulness of the update, but I never bothered going from 3.6.18, because the site did not say that it was an update for PCs.
But, I had an interesting time, having installed the final(?) version of Firefox 6, which installed in the default location, right on top of 3.6.18. So, when this morning I tried to use 3.6.18, the browser went through all sorts of compatibility tests, indicating that something was wrong. I stopped the loading operation, and, immediately, went to the Add/Remove. Only Firefox 6 was there, although the old beta was elsewhere. So, I renamed the old FF 6 beta to 3.6.18, and reinstalled 3.6.18. In each case, I made sure that I used the correct profile. Then, on reading your post, I was that there was an update, which I installed via Help|Check for Updates. In spite of this nonsense, it, probably, took me less time to get back to normal, than downloading a new file, installing, and all that.
To summarize, my FF 6 is installed where 3.6.18 was, yesterday, and 3.6.20 where FF 6 beta was yesterday.
And, I have two Mozilla Firefoxes in my Add/Remove. Which proves my point held for long time, that it is vital to keep the Profiles in a row, but not the Programs.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:126.96.36.199) Gecko/20110803 Firefox/3.6.20