Style editor traffic in the Developer Tools can be routed through a service worker hosted on a third party website if a user selects error links when these tools are open. This can allow style editor information used within Developer Tools to leak cross-origin. This vulnerability affects Firefox < 58.
https://www.mozilla.org/security/advisories/mfsa2018-02/
https://usn.ubuntu.com/3544-1/
https://bugzilla.mozilla.org/show_bug.cgi?id=1408708