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 to be 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.

If you do not want to set that page as the home page then you can append the URL to the command line of the Firefox desktop shortcut.

• https://developer.mozilla.org/Command_Line_Options