The way that Firefox caches intermediate CA certificates could allow for the fingerprinting of users and the leaking of browsing details. A security researcher based in Germany, Alexander Klink, discovered the issue and reported it to Mozilla in January but details of the bug weren t made public via the company s Bugzilla platform until Tuesday. Klink discovered that through a third-party request in this case one sent from his proof of concept website he could determine which intermediate CAs a Firefox user has cached, even if the user is operating the browser in Private Browsing mode.
Source: https://threatpost.com/intermediate-ca-caching-could-be-used-to-fingerprint-firefox-users/123834/