With native XHR:

Using native XHR, Chrome seems to always have a first preflight request failing, with a second one being successful. This doesn't happen with AJAX. Not sure if this is helpful in any way, but could this explain why requests keep failing at all in Firefox, and how to resolve it?

CodePudding user response：

After some more research I finally found out that this is actually a firefox-related issue with userscripts: https://bugzilla.mozilla.org/show_bug.cgi?id=1715249