buddy, looks like this is not the problem of firefox. it is some malicious website tricks. you have got their "award". well, do some antivirus, and completely remove firefox, and do a clean installation of firefox 4.0 using firefox install pack from mozilla official, if this yet don't help, then i strongly recommend you do some backup and reinstall your windows to completely remove those dirty things you get in you operation system. it is really really time wasting for you to remove them, sometimes reinstall is much faster and easy. and if you really like those malicious sites, maybe linux, like ubuntu/mint/pclinuxos is a better choice.

I think I solved it.

I searched my extensions folder (in my profile) for all image files (PNG and JPG), and found the pop-up images to be in an extension folder related to Firefox Sync (not an external extension, actually). But even though I had already disabled it and un-linked this computer from Firefox Sync, the message still popped-up.

I think the image filename was SYNC.PNG, or something like that. Close Firefox, find the folder where that image is in, and rename the main folder (the one right under the extensions folder) to some other name (such as "original name_copy"). Then start Firefox. Check if it works OK and if the pop-up ceases to appear. If everything OK, then you can delete that folder.

If something goes wrong, just rename the folder to what it was before.