See these articles for some suggestions:

• Firefox has just updated tab shows each time you start Firefox
• How to set the home page - Firefox supports multiple home pages separated by '|' symbols
• http://kb.mozillazine.org/Preferences_not_saved

Ah, I figured out the answer after reading the first article. It wasn't set as my home page because that would open in a separate tab at the same time. But the talk about Preferences got me thinking. It is happening because I'm using both Firefox 3.6.16 and TenFourFox 4.0.1 on my G5 iMac. T4F 4.x is a PPC version of FF 4.x so it is changing the settings file to reflect that I'm using a version of Firefox 4. However, Firefox 3.6.16 isn't smart enough to recognize that Firefox 4 is NEWER! Thus when I happen to launch FF3.6.16, it thinks it is the latest version and pops up the "updated" tab.

If you use two different Firefox versions then you should create and use separate profiles for each of them.

See this article about how to install multiple versions on Mac OS X:

• http://www.businessinsider.com/how-to-run-firefox-4-and-3-simultaneously-in-mac-os-x-with-multiple-firefox-profiles-2011-3