I have attached a revised version to this post. While Frefox is closed replace the old copy of that file with the one I created.
It might be best to hold one to the old copy in case anything goes wrong.
Then open Firefox and
- Type about:config in the URL bar and hit Enter.
- If you see the warning, you can confirm that you want to access that page.
- Then navigate to these entries by searching for browser.startup.homepage_override
- Right-click each one and select Reset.
Then reboot your computer and open Firefox agaion and let me know if it stuck.
If it comes back it would be very helpful to me to know what triggers the reset.