Type about:config<enter> in the address bar. If a warning screen comes up, press the I Accept the Risk button. At the top of the screen is a search bar. Type browser.startup.homepage

What is the value? Right-click and select Reset to restore its default value; about:home

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

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

Note that a new window by default opens the home page(s).

You can check the home page setting.

• Options/Preferences -> General -> Startup: Home page
• https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Firefox supports multiple home pages separated by '|' (pipe) symbols.

In Firefox for Windows, Ctrl+Shift+n re-opens the last closed window. So you should recognize the page, unless it was a pop-up that was closed quickly or automatically so you never noticed it.

Next time, instead of using Ctrl+Shift+n, go to the History menu and look at the list of Recently Closed Windows to see what it is that you want to re-open. You can access that using either:

• "Library" toolbar button () > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows