Certain versions¹ of Firefox support
Before Firefox 44 (which added
Headers), there was no non-brute-force way to find out what set of headers were actually set in the object. The information simply isn't exposed anywhere.
That said, releases prior to Firefox 45 (the current ESR release) are all out of support and have known security bugs, so users should really not be using those. I don't think it's worth catering to users on those releases; they should be urged to update instead.