Hi This is not an issue with Firefox for Android, but is from a site that you are or have visited. I strongly recommend that you do not follow the instructions in the message - it is a scam and will result in lost data or damage to your device. The best way to remove this is to clear the browing history - this article will help: https://support.mozilla.org/en-US/kb/clear-your-search-history-firefox-android. Further to this, I suggest keeeping an eye on which site you visit if it reappears, and avoid that site in the future.