Firefox won't change your home page. It may open up a new page as needed (called a whatsnew page) on your first restart after an update, to show what's new and changed, but that is it. Your home page preferences are always respected.

Couple questions:

• What is the new home page you got after the last update?
  • Does it appear every time you use Ctrl+n to launch a new window, or only when you start Firefox using a Desktop or Taskbar shortcut?

• Did you notice any other changes?
  • For example: changes in which toolbars are displayed, or what extensions are installed.

That information would help in figuring out why it did not work as designed.

since I reset it I'll have to remember when it does it next time.

Some programs can change things like user settings without the users okay.

Bookmark and use this; Mozilla Search Reset {web link} This add-on is very simple: on installation, it backs up and then resets your search preferences and home page to their default values, and then uninstalls itself. This affects the search bar, URL bar searches, and the home page.

Settings Guard for Firefox {web link} Detects and resets changes to settings that are frequently done by add-ons and application installers.