Windows keeps a copy of all of your picture, video, and document thumbnails so they can be displayed quickly when you open a folder. The thumbnail preview problems can be resolved by clearing the thumbnail database and let the system rebuild it as and when required. To do so, use these steps:
Clearing the Thumbnail Cache using Disk Cleanup
1. Click Start, type cleanmgr (or type disk cleanup) and press {ENTER}
2. Click My files only (You may not see this option if you’re using Windows 7)
3. Select the system drive letter from the drop-down.
Uncheck all the entries, except Thumbnails
Click OK, and click Delete Files to confirm.
More Information
The thumbnail database is stored globally in Windows 7 & Windows Vista, unlike Windows XP, where the thumbnail database is stored on a per-folder basis. In Windows 7 & Windows Vista, the default thumbnail cache location is mentioned below:
When you open the Windows Vista Start menu and right-click Recent Items, the option Clear Recent Items List may be missing. Also, the Recent folder may be displayed with a generic folder icon in Windows Explorer. 
If you find Mozilla Firefox browser missing in the Set Program Access and Defaults (SPAD) dialog, here is a fix to restore the entry back. 