You may have ad/mal-ware. Further information can be found in this article;

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

What's your computer system and Firefox version?

Is your issue with a Firefox desktop or laptop?
Because that forum is where you posted from an Android device.

User Agent: Mozilla/5.0 (Android 10; Mobile; rv:68.0) Gecko/68.0 Firefox/68.0

Different forum for Android support. A Moderator here can move this thread to the Firefox for Android support queue.