Archlinux.com has good tips for getting Firefox faster and lihter, but there is some externsion and settings in Firefox which allowss webpages show grayscale and black and white.
From the Firefox´s settings
about:preferences -> Colors -> Manage colors -> underline links always
And with that modification there is a good extension Grayscale Browsing
Read also: https://puolanka.info/goto/how-to-make-firefox-load-sites-faster/