Firefox not deleting HTTP Basic authentication credentials although being instructed to do so

Firefox (Firefox 68.0, under Linux) doesn't store this info in Cookies and Site Data but in History.

You need to go to Preferences->Privacy & Security and under the History section, click on Clear History.... Make sure the Active Logins checkbox is checked before you submit.

The accepted answer (clearing "Active Logins" from "Clear History...") has the disadvantage that it clears all active logins, including from other sites on other browser tabs.

An alternative, at least with Firefox, is to reload the page by specifying a different (or invalid) login. This can be done by adding it in front of the URL:

http://[email protected]/...

This can also be used to login as a different user this way:

http://other-user:[email protected]/...

While such URL's have been deprecated in RFC 3986 for security reasons, they still work in Firefox v. 80. So it's still a viable workaround until browsers implement better authentication management.