I asked a similar question here about night shift. When everything’s orange and less bright it makes the white easier.
Depending on your use case for a disposable firefox, you can consider installing an extension. Since your IP with firefox is revealed, as is screen size, and other things that vanilla firefox might not hide, fingerprinting and tracking is much easier. Installing an extension probably wouldn’t be the difference between an anonymous fingerprint and a unique one. In Tor Browser it would, since Tor does much more to protect against fingerprinting.
Also if you view bright white PDFs you can use the zathura pdf viewer, and press Ctrl+R (IIRC) to invert the white background and black text.