Click the 'hamburger' (three horizontal lines) at the top right of your screen. Click 'Add-ons' and then 'Extensions'. See if there's any unwanted extension that's found it's way there. If there is, click 'Remove'. Close the tab.
Then (irrespective of whether you found anything there or not) go to the web page that you want Firefox to use as your home page. Click on the 'hamburger' and then on 'Options'. If 'General' isn't already pre-selected, click on it. Click on 'Use Current Pages'. Close the tab.