Videos keep playing after closing browser

Videos continue to play even after I have closed the browser app and you can still hear the audio even if I've locked my phone screen. This happens with adverts on webpages, the follow-on videos after watching a tik tok link, anything. It's incredibly annoying as the only way to prevent it is to make sure I close the tab with the video before closing the browser - if I forget then I have to unlock my phone, reopen Firefox and close the tab, all while the audio is still playing. I want all video to stop playing if I close the browser or lock my phone. How can I fix this?