Although it is rare, photos, music, and video files can be specially hacked to attack software on your computer. It's more common with document files like PDFs. This is why software vendors have to keep releasing updates, because people keep discovering flaws that allow such attacks. I don't think Firefox Sync actually moves any of those kinds of data between your different installations of Firefox. You can control what is synchronized from the settings on the Options page. See: [[How do I choose what types of information to sync on Firefox?]]