How to fix the wrong-start-page-problem when occurring with Firefox browser

Recently, my Firefox browser always opened with a start page (or startup page) displaying content from a third-party provider I never had invited to do so.

I fixed this problem in three quick steps:

1. Open the desired start page in sabotaged Firefox browser.
2. Select and drag the URL string from the address bar to the Home button in the toolbar. 
3. When asked “Do you want this document to be your new home page?”, click Yes.

Now, my desired start page comes up each time, when I launch Firefox.

Note: I experienced some confusion about the terms “home page” and “start(up) page” when consulting different tutorial or support websites. My understanding is that one may set one's home page as a browser start page, but does not have to. Personally, I like to have a search engine page as my browser start page; neither my home page nor a browser-enforced page.