WebExtensions with the "ActiveTab" permission are able to access frames hosted within the active tab even if the frames are cross-origin. Malicious extensions can inject frames from arbitrary origins into the loaded page and then interact with them, bypassing same-origin user expectations with this permission. This vulnerability affects Firefox < 58.
https://bugzilla.mozilla.org/show_bug.cgi?id=1396399
https://usn.ubuntu.com/3544-1/