A couple of things to check.

Check to see if Firefox is set to open to your home page, if so have the sites that open been set as the home page. Those settings are on the Startup, home page, tabs, and download settings.

Another possible cause is the file that stores details of sessions may be corrupt. You can delete the files that have details of sessions to see if that helps. Firstly you need to access your profile folder, for details of how to do that see the profiles article. Once you are in your profile folder, close Firefox and delete sessionstore.js and sessionstore.bak (the second file may not exist)

I had the exact same problem and I double-checked under System Preferences to see what I had set my home page at and it had gotten set to the multiple pages I had opened at the time I originally set my home page. I closed all the tabs, opened just my preferred (single) home page, re-set the home page to that and that solved the problem. Hope this makes sense and hope it helps someone!