https://bugzilla.mozilla.org/show_bug.cgi?id=1378626
1378626 - Intermittent browser/base/content/test/general/browser_beforeunload_duplicate_dialogs.js...
RESOLVED (nobody) in Firefox - General. Last updated 2018-10-01.
intermittentbrowserbasecontent
https://bugzilla.mozilla.org/show_bug.cgi?id=1567738
1567738 - Fetch API not working in beforeunload
RESOLVED (nobody) in Core - DOM: Networking. Last updated 2019-07-23.
fetch apinot workingbeforeunload