Comment by dessant

Browser vendors don't take clearing browser data seriously, see how Firefox has implemented the browsingData extension API [1]. These bugs compromise the security and privacy of Firefox users, but fixing them has not been a priority over the years.

Built-in clearing options in Firefox will also leave classes of cached data behind. The only reliable way to wipe everything has been to delete specific files from the Firefox profile folder before the browser launches.

[1] https://armin.dev/blog/2019/03/firefox-extensions-browsing-d...