How do I stop/delete Secure PC Cleaner/guardtowerstag.com

Every time I hit the home page button, this webpage also opens http://ic-dc.guardtowerstag.com/pr/fbea5fa6-6530-11e6-86b1-0695da005429/typ%202. html. It says it's for Secure PC Cleaner and thanks me for downloading. I never downloaded this purposefully, so I don't know how it got on my computer or Firefox. How do I stop and get rid of this annoying problem?

Thanks!