You could possibly use code in userContent.css to disable ligatures.

• https://developer.mozilla.org/en-US/docs/Web/CSS/font-feature-settings
• https://developer.mozilla.org/en-US/docs/Web/CSS/font-variant-ligatures

More info about userChrome.css/userContent.css in case you are not familiar:

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html
• https://www.userchrome.org/firefox-changes-userchrome-css.html

You need to set this pref to true in about:config to enable userChrome.css and userContent.css in Firefox 69+.

• about:config => toolkit.legacyUserProfileCustomizations.stylesheets => true
• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

I'm talking about pages authored by others, not myself.

You can override CSS rules supplied by the webpage with your own rules in userContent.css by appending the "!important" flag to each property value.

• https://developer.mozilla.org/en-US/docs/Web/CSS/important

Thanks. I've never written a CSS so I took a minute to figure it out. For those as slow as I am:

body{

   font-variant-ligatures:	none; !important

}

in chrome/userContent.css

It annoys me that it's in the 'chrome' subdirectory, a subdirectory I didn't have until now. Why doesn't Firefox have its own?

Firefox uses the "chrome://" protocol to access its internal files and use chrome in other cases like userChrome.css/userContent.css (no relation with Google Chrome).

• https://developer.mozilla.org/en-US/docs/Chrome

Thanks. Chrome seems like a poor choice of words. What is it supposed to mean? And why use ligatures inappropriately? Many instances of ff and ffi in English aren't ligatures - one knows because the word should be separately into syllables by dividing them there; one can't do that with 'real' ligatures such as æ and œ.