You can check the target line in the Firefox desktop shortcut (right-click: Properties) to make sure nothing is appended after the path to the Firefox program.

• https://support.mozilla.org/en-US/kb/wrong-home-page-opens-when-i-start-firefox-how-fix

https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

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