http://support.mozilla.com/nl/forum/1/583132

Recently we've been having a lot of customers coming in for a check up on their computers due to a "webpage" or a number of them that open up even if they have their browsers closed... some are plain text websites; most of them are related to stocks and investment opportunities. if you notice tabs or browser windows opening without you requesting them please update your AV software, if you don’t have one install some sort of malware and spyware removal tool (malwarebytes and spybot are the ones we recommend) you should also update both software and run the AV, and both the malware and spyware removal tools, if this doesn't make any changes please give us a call and we'll try to fix the problem for you.