To remove background images using js rather than a stylesheet, while keeping the rest of the styling, in versions of firefox that do not implement StylePropertyMap:
(removeprops => {
const clean = (csd, plist) => plist.forEach (p => csd.removeProperty (p))
const ondecl = csd => clean (csd, removeprops)
const onrules = rlist => Array.from (rlist).forEach (onrule)
const onrule = r => {
if ("style" in r) { ondecl (r.style) }
if ("cssRules" in r) { onrules (r.cssRules) }
}
Array.from (document.styleSheets).forEach (sh => onrules (sh.cssRules))
Array.from (document.querySelectorAll ("[style]")).forEach (e => ondecl (e.style))
}) (["background-image"])