NGINX - CORS error affecting only Firefox
Solution 1:
the problem was happening because Firefox didn't authorize the API's SSL cert. Trusting the site's cert by navigating to the endpoint with Firefox solved the issues temporarily, while changing the cert - permanently.
Header issues with Firefox and LimeSurvey remotecontrol API can be fixed by proxying fixed header values, or sending blobs, as per https://stackoverflow.com/questions/24465304/trouble-changing-request-headers-in-firefox-with-angularjs