I finally figured out why Firefox sometimes decides to run my search on a non-default search engine. It only happens sometimes, and I was worried it was due to some kind of malware. But it's much simpler than that--it's all about where your mouse pointer happens to be when you hit Enter. Behold:
Here I've typed "electron" as my search term, and the autocomplete drop-down appears to show related searches. It correctly shows Google as my default search engine. But note the blue highlight there near the bottom. That's the DuckDuckGo search engine that's highlighted. But how did it get highlighted? By a bump of my mouse, which stole focus from the search box and placed it on the DuckDuckGo icon, over which the pointer happened to be hovering. I didn't make the connection until today, when hitting Enter didn't search at all, but rather opened the Search Settings in the Options window.