Apparently Firefox cannot verify xkit. Time to swap back to Chrome.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste xpi and pause while the list is filtered

(3) If the xpinstall.signatures.required preference is bolded and user set to true, double-click it to switch it to false

Note that this allows unsigned extensions to run in your Firefox! You want to pay very close attention to your extensions while this is turned off. When NewXkit gets their extension approved again, turn this back on!