Make sure that you haven't set the home page or new tab page to a page that downloads that DMG file. Try to reset some preferences to the default with the SearchReset extension: *https://addons.mozilla.org/firefox/addon/searchreset/ Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons). ----- Make sure that Firefox gets installed properly. Open the Firefox disk image (DMG) file and drag the Firefox program to the application folder on your hard drive.<br /> You shouldn't double-click the Firefox application to run it from the disk image, but drag it out of the DMG folder instead. ---- *https://support.mozilla.org/kb/Installing+Firefox+on+Mac *http://kb.mozillazine.org/Installing_Firefox#Mac_OS_X