"Clear Flash Cookies" is a Firefox extension that automatically deletes Flash cookies ("Local Shared Objects") after Firefox exits, like the defunct "BetterPrivacy" add-on. Flash cookies can be used to create "super cookies" that recreate deleted HTTP tracking cookies. To view your existing Flash cookies, open Adobe's Flash Storage Settings panel.
Warning! Clearing Flash cookies may delete saved game data from Flash games (e.g. some Facebook games).
Flash cookies are less of an issue in Firefox 55+ because Flash is click-to-activate by default, but deleting Flash cookies is a good precaution. If you don't actually need Flash, consider disabling or uninstalling it: https://support.mozilla.org/kb/disable-or-remove-add-ons#w_how-to-disable-plugins
Please feel free to send me new or corrected translations in email or on GitHub. The code is licensed under the Mozilla Public License Version 2.0. The cookie icon was created by W. X. Chee.
To be totally accurate, this extension does not delete Flash cookies during Firefox exit, but when Firefox next launches or when you close all your Firefox windows. But the effect is the same: no Flash cookies from a previous browsing session will be persisted to your next session.