Development Tools panels of an extension are required to load URLs for the panels as relative URLs from the extension manifest file but this requirement was not enforced in all instances. This could allow the development tools panel for the extension to load a URL that it should not be able to access, including potentially privileged pages. 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=1425224