Cookie 5 doesn't work at all for me with Firefox 68.0.2 on El Capitan (10.11.6). Firefox shows cookies in Privacy & Security >> Cookies and Site Data >> Manage Data but Cookie 5 only shows extensions. I have Cookie 5 set to delete on browser quit and have tried the things listed in this thread. The option to keep cookies in Firefox "Till they Expire" is not present in this version (or I can't find it). Is there anything else I can try?
@sweetp thanks for the confirmation. I had issues different from what @alohart was having, and I'm having other issues with Big Sur so I'm going back to Mojave for now. Hopefully I can upgrade to Big Sur (and Cookie 6) at some point.