I have had this same problem repeatedly over the last couple of years. Every time I fix it it recurs again a month or two later. I have read every article online that matches, followed all advice and still the problem comes back. This time I can't even fix it: It starts with Firefox preventing Macbook Pro from shutting down (OS 10.11.6 - late 2011 model). It won't force quit I have cleared the library and the cache. I have uninstalled and removed the whole Firefox folder from Library/APplication Support. The macbook then shuts down properly. I have re-installed firefox. BUT without even opening Firefox just putting it in the applications folder, the mac now won't shutdown - the desktop and top menu bar disappear and the wallpaper just hangs. I have run Malwarebytes to see if I have some malware issue - but it's come up clean. Any suggestions gratefully received! Speaking to you from Safari! Thanks S