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

https://addons.mozilla.org/en-US/firefox/addon/grayscale-browsing/

Read also: https://www.puolanka.org/goto/how-to-make-firefox-load-sites-faster-via-custom-css/