A use-after-free vulnerability can occur when the thread for a Web Worker is freed from memory prematurely instead of from memory in the main thread while cancelling fetch operations. 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=1418074