When visiting many web sites, especially sites that I expect not to visit again, I usually use a tab in a private browsing window in Firefox. More and more frequently, when I look at cookies (in a normal browsing window), I find cookies from sites that I visited only in a private browsing tab.
In a bit more detail, I go to Firefox > Preferences > Privacy & Security > Clear Data and I clear Cached Web Content. Then I go to Firefox > Preferences > Privacy & Security > Manage Data and see references to the sites that I visited only in a private browsing window.
Why is that happening and how can I prevent it? Thanks.
Edited to add: I just noticed that one site, in Firefox > Preferences > Privacy & Security > Manage Data, was shown as having no cookies but did have Storage. Since I had cleared Cached Web Content but not Cookies and Site Data, I assume the Storage was Site Data. What is that? (I still think other sites are saving cookies, but I’m no longer as sure.)