In the Privacy & Security pane we have the following configuration Clear history when Firefox closes with all items selected except for Site Settings. Delete cookies and site data when Firefox is closed with some exceptions in Manage Exceptions. But after quitting and re-launching Firefox there frequently entries for random websites shown in the "Manage Data..." pane. This seems to related to having Facebook Container and Firefox Multi-Account Containers extensions installed. It does not seem to happen when they are not present. Interestingly. we had been using a a very similar Firefox configuration on a legacy Intel macOS system without this issue. We began seeing it while setting-up a fresh installation of Firefox on a new Apple Silicon system with macOS Ventura. The issue was reproduced in a new profile. It continues to the present day with up the current version of Firefox 129.01 on macOS 14.6.1 I feel like this is probably a bug but I don't have the necessary skill to make an effective bug report.