That would make links even harder to distinguish. None of these actually make everything strictly black and whitw

https://www.squarefree.com/bookmarklets/zap.html see/click on "zap colors" leaves images alone, leaves links with some color.

Black text on white background - userstyles.org
http://userstyles.org/styles/554/black-text-on-white-background

A button that you can toggle: Page Colours http://codefisher.org/toolbar_button/toolbar_button_maker

You can use the NoSquint extension to set font sizes (text/page zoom) and text colors on web pages.

• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

This thread has some more ideas for styling pages with white text on a black background, if that's relevant. With a little bit of color so you can see buttons.

Ink wasting because pages with dark backgrounds still print despite background print checkbox being unchecked | Firefox Support Forum | Firefox Help