perhaps a software such as Adaware could help you here?

You might be infected. But also I find that the pop-up blocker with the google toolbar works quite well.

1) The fact that you *use* Firefox as your internet browser does not guarantee that Firefox is you *default* Browser. You could just start Firefox from the Firefox icon every time you want to use the internet browser. But if Firefox is not your default browser, some applications which use you default browser could still use Internet Explorer. 2) "Your default browser is opened automatically when you click on a URL in a mail application (like Thunderbird or Outlook), an Internet shortcut, or an HTML file. This article will show you how to make Firefox your default browser." "Note: MSN Messenger and other applications may be created to open Internet Explorer, no matter which browser is set as the default. Also, Internet service providers like PeoplePC Online, Juno and NetZero may provide connection software that automatically launches Internet Explorer." "1. Open Firefox. 2. At the top of the Firefox windowOn the menu bar, click on the ToolsFirefoxEdit menu, and select Options... 3. If it isn't already selected, select the Advanced panel, then click the General tab, and then click Check Now. 4. Select Yes to set Firefox as your default browser. * If Firefox still isn't the default browser, see Setting Firefox as the default browser does not work. To change Firefox to no longer be the default browser, you'll have to set another browser to be the default." Source and further information: http://support.mozilla.com/en-US/kb/How+To+Make+Firefox+The+Default+Browser