how I made Firefox use two starting pages at once
 

I accidentally discovered a strange but interesting practical use for a pipe (|). Recently, I changed my Firefox's starting page. Since then, whenever I open a new Firefox window, two tabs automatically open--the starting page and the search page at my local library (which I had bookmarked). I hardly noticed that at first, but eventually I checked Preferences-->Main. Apparently, it was my intended home page and the library's page, separated by a pipe.
So I understand why I had two starting pages, but have no idea how they both got entered; it's a weird curiosity. It could only be an error, because I didn't do it on purpose and I wouldn't have figured out how. (The computer and especially Firefox have been behaving strangely, so I can see this "just happening.") I haven't used pipes often enough to realize all their possibilities.

You may have had two tabs open and then selected to use them as your start point. You can do this with multiple tabs - not sure if there's a limit.

If so, it was an accident.