Add browser.helperApps.deleteTempFileOnExit to our profile

from the almighty AF

/* 2603: remove temp files opened with an external application
 * [1] https://bugzilla.mozilla.org/302433 ***/
user_pref("browser.helperApps.deleteTempFileOnExit", true);

cc: @pierov

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information