Hello,

Can you please confirm if you are viewing these videos on Youtube? If so, then I believe, these ads are part of Google's offering to show ads under the videos. If this is not youtube where you are seeing this issue, can you please let us know which site this is, so that we can suggest a possible way out.

Thank you

Did you install a program called FlashFox? It is not made by Mozilla. It contains extra advertizing that creator adds.

You can install Firefox from the Google Play store, https://play.google.com/store/apps/details?id=org.mozilla.firefox and installing Flash from Adobe https://support.mozilla.org/en-US/kb/how-do-i-watch-flash-videos-firefox-android

If this is not the case then please provide the site which you are having an issue with.