This was removed by Bug 1853323, apparently it didn't work consistently anyway.

Equivalent userContent.css:

*|*:any-link { 
  text-decoration: none; 
}

It is still there in ua.css

• https://searchfox.org/mozilla-release/source/layout/style/res/ua.css

:any-link {
  cursor: pointer;
  text-decoration: underline;
}

You do not want to force it for all cases like SVG links.

• https://searchfox.org/mozilla-release/source/layout/svg/svg.css

a:any-link {
  /* We don't want SVG link to be underlined */
  text-decoration: none;
}

See also:

• 1664278 - Feature request: “Always underline links in text” option in Firefox

Thanks zeroknight and cor-el.

The solution presented by both of you solved the issue through CSS.

To elaborate on cor-el's solution, for those who have no experience with the userContent.css, here is how you now change your preference to not underline links by default:

Go to "about:config" Toggle "toolkit.legacyUserProfileCustomizations.stylesheets" to "true"

Go to "about:support" Look for "Profile Folder" Click "Show in Finder" (on Mac), "Show Folder" (Windows), "Open Directory" (Linux) or "Open Containing Folder".

In that profile folder, look for "chrome" folder, create if not present

In "chrome" folder, edit or create "userContent.css" file, put this in there:

 *|*:any-link {
   text-decoration: none;
 }

Restart Firefox

I need more help. Where do I find about:config? I am using MacOS 13.5.2 and Firefox 119.0.1. And why is it so hard to get rid of the underlining? The ability to disable link underlining is why I first started using Firefox many years ago. Thank you.

The easiest way is to install the extension "Default links not to be underlined" made by a Firefox developer.

Note that Firefox 120 will have a pref to force underlining of links and Firefox 121 exposes this pref in "Settings -> General -> Browsing: Always underline links", so make sure you do not enable this setting.