Limitations on the URIs allowed to WebExtensions by the browser.windows.create API can be bypassed when a pipe in the URL field is used within the extension to load multiple pages as a single argument. This could allow a malicious WebExtension to open privileged about: or file: locations. This vulnerability affects Firefox < 64.
https://www.mozilla.org/security/advisories/mfsa2018-29/
https://usn.ubuntu.com/3844-1/