Hi Ken, one possible reason for this is if the Preferences page was added as a second "home tab" in which case it will show up in every new window (Cmd+n) and anytime you click the Home button.

To check on that, you can use the Preferences page, General panel, Startup section, as described in this article: Startup, home page, tabs, and download settings.

In the Home Page field, look for a vertical bar character separating two addresses. As a simple example with short URLs:

https://www.mozilla.org/|about:preferences

The bar definitely can be hard to spot with long URLs, but when you find it, delete everything to the right of the bar and then the bar itself (I suggest deleting the bar last so things don't accidentally get mushed together).

Was that it?

Note that this can happen if you click "Use Current Page(s)" to set the home page. You can pin the tab with the about:preferences page to prevent including about:preferences to the home page setting.

Thanks guys that was exactly the fix. In the Preferences/Home Page, there was my correct home page, then the bar, then about:preferences, after deleting the bar plus about:preferences, my Home Page comes up in a single tab. I appreciate your help. Ken