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.

This is something that happens but this is rare.

I don't think either response is to the point. It's FIREFOX upgrading that seems to step all over the home page setting, not gremlins nor add-ons.

It's a bug in the upgrade process. I don't know how to respond to "this happens rarely">

I filed a bug report some time ago;

https://bugzilla.mozilla.org/show_bug.cgi?id=1031347 Firefox Full Installer Changes User Settings

Now that's an answer. While we're here, what else besides home page might it step on until the bug is fixed?

There are some programs and even installers that change settings without getting permission. Some by trickery.

It’s very sad, but many of the software down-loaders / installers will trick you into installing not only their program, but other programs as well. You have heard of the fine print in shady contracts, right? Well, some installers you need to look at the itsy bitsy teeny weeny fine print. You are thinking you are giving the installer permission to install the program you want by using the recommended option. But if you use the Manual Option Instead, you discover all kinds of stuff that you do not even know what it is or what it does. From now on, everyone needs to Use The Manual Option to put a stop to this.

Which is what I generally do, though to stick to the point, we're talking here about the FIREFOX INSTALLER, which doesn't have such options and shouldn't step on any user settings. So to ask the question once again:

What else (besides home page) might the Firefox upgrade installer step on?

I can not say. I know that my proxy setting was changed once. I don't know what others have had.