the popup only opens when something was clicked on that facebook says could be malware. If you try another browser and it does not appear, then we may be able to get rid of the problem without scanning. If you have the exact same problem on another desktop browser, then it could be something major that normal malware scans will not find. As far as logging on with other devices. There is a big difference between moble devices browsers and computer devices as far as the way pages appear.
One thing you can try to do is to delete your browsers cache and cookies and restart the browser.
Remember you are asking for help because you don't have the answer. We are not sitting in front of the computer so sometimes suggestions are made in order to give clues as to what is causing the problem. It is up to you to try the suggestions. however it is also up us whether or not to help any further if we cannot get valid and valuable information in order to help.