If you want to turn off the picture-in-picture mode option in Firefox, then you do this by accessing the settings of the Firefox browser. Here are the steps:
1. Open the Firefox browser
2. Click on the hamburger icon (three horizontal lines) available on the top right corner of the Firefox browser. This will open the Application Menu of Firefox
3. Click on the Settings option. Firefox Settings tab will open with the General menu
4. In that menu, scroll down the page to access the Browsing section
5. There you will see a toggle to Enable picture-in-picture video controls option
6. Uncheck the toggle to turn off picture-in-picture mode.