I have recently begun experiencing unwanted app ads on my Nexus 4 phone. Firefox automatically opens when I unlock my phone or at other times (can't pin it down to one app... Seems random), and displays ads for games or apps, the most common being Uber (ubermeasure). The back button gets me to a URL that seems to be a 3rd party marketing site. Is there any way I can block this site or the ad sites?