Does that happen when you start Firefox or when you have Firefox running and open a new window? What are your history settings on the Privacy & Security page of Preferences? Do you ever clear recent history (ctl+shft+del).

You can check that those pages aren't set as multiple homepages.

You can check the homepage setting.

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

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

You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data.

• rename/remove sessionstore.jsonlz4 and sessionstore.js

You may also have to rename the sessionstore-backups folder.

When sessionstore.jsonlz4 is deleted you lose pinned-tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them. sessionstore.jsonlz4 is only present in the Firefox profile folder when Firefox is closed.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data