https://www.ihaveapc.com/2011/09/how-to-delete-flash-cookies-from-firefox-in-linux-mint-ubuntu/ I just checked that hidden folder and it is now empty. THis is a simple command and sure you can just to the folder in a file manager and just select all and delete them but if your worried about your privacy this is another weapon in your arsenal. They also have a script written for none programmers like me with the same command you can put on your desktop and execute when ever you think about it.