Type about:preferences#general<enter> in the address bar.

There are two settings;

• When Firefox Starts

When the browser is started, what do you want to display. Many of us choose Show My Home Page.

• Home Page

When a new page is opened, what do you want displayed.

1. Use Current Page. Use what ever page(s) are open at that time.
2. Use Bookmark
3. Restore To Default. about:home shows a Mozilla home page with tools.

You can use any of these that you wish;

about:home (Firefox default home page), about:newtab (shows the sites most visited), about:blank (a blank page),

or you can enter any webpage or about: page you want.

My preferences are already set as you suggest. My problem is that, as well as the page I want to appear when I start Firefox, a separate page also opens up in addition to the page. The second, separate page, the one about Media Extractor, is the page I don't want to open up.

It isn't that the Media Extractor page replaces the start page I want, it is that, as well as the start page I want, a completely separate page for Media Extractor opens up as well. I then have to remember not to click to install this addon I don't want and then close the unwanted media Extractor page.

Did you check the homepage / start page settings?

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.