Summarize:

The Firefox app icon resembles an orange fox wrapped around a blue globe. It's in the top-right of the Firefox window. Doing so opens a drop-down menu. You'll find this option near the bottom of the menu. It's the option near the middle of the drop-down list. This is in the top-right side of the troubleshooting page. Doing so will prompt Firefox to "refresh", which involves removing any installed add-ons (whether or not they were installed by you) and resetting Firefox to default settings. If the problem persists, repeat this process but select Restart with Add-ons Disabled instead. If this fixes the issue, you'll need to remove any add-ons from Firefox. You may need to do this to prevent Bing from appearing as your home page even after removing problematic add-ons:  Click ☰  Click Options (PC) or Preferences (Mac). Type a website address into the "Home page" text box or click Restore to Default
Open Firefox. Click ☰. Click Help. Click Troubleshooting Information. Click Refresh Firefox…. Click Refresh Firefox when prompted. Consider changing your homepage on Firefox.