One alternative solution would be to investigate some of the add-ons available. Some of these may need a bit of setting up but then modify how websites display.

• https://addons.mozilla.org/en-US/firefox/addon/nosquint/

About this Add-on
Having a hard time reading on the web? Is that website with the ridiculously small font and hot-pink-on-slightly-darker-hot-pink text raising your blood pressure? NoSquint can:

   Override the default text-only and full-page (both text and images) zoom levels for all websites
   Enforce your own foreground and background colors
   Remember your zoom levels and color settings per site, automatically applying them when you return.
   Disagree with what NoSquint calls a site? A powerful exceptions mechanism lets you split up or group together sites with URL patterns.

A second alternative if you are good with the keyboard is to use keyboard existing shortcut sequences. I do not know them offhand for Windows, but they are not likely to be particularly convenient unless well learnt and used frequently.

{Alt} - then - {T} - then - {O} will open Options from the keyboard.

You can also consider to look at the PrefBar extension that has some buttons to make such changes and you can create your buttons to change font settings and toggle specific prefs.

• PrefBar: https://addons.mozilla.org/firefox/addon/prefbar/

• browser.display.use_document_colors
• browser.display.use_document_fonts

• font.minimum-size.x-western

This reply is especially helpful, thank you.