Override style per web site in Firefox via userContent.css

userContent.css

Sometimes, the font size on a web site is too small, for example at https://www.retsinformation.dk/ where the font-size is 11.2 pixel. You can override the text size locally, via the userContent.css file.

Add a file named userContent.css file inside the Firefox profile folder called chrome (~/.mozilla/firefox/ID_STRING.default/chrome) which you might have to create. Then add this:

@-moz-document domain(www.retsinformation.dk) {
  p {
    font-size: 18px !important;
  }
}

.. and restart the browser. The p font at https://www.retsinformation.dk/ should now be 18px.

If it doesn't work, check that toolkit.legacyUserProfileCustomizations.stylesheets is enabled in about:config.