Some visited addresses are stored inside notificationstore.json in the profile folder
Might be related: https://bugzilla.mozilla.org/show_bug.cgi?id=1095073
The file persists after closing the browser.
Visiting the following address can create one if it doesn't exist:
https://privacycheck.sec.lrz.de/active/fp_fd/fp_feature_detection.html
The file will get bigger after each visit. That doesn't seem secure.
Edited by cypherpunks1