Firefox 40+ [underlines <abbr>](https://bugzilla.mozilla.org/show_bug.cgi?id=1157083) elements instead of using a bottom border. (This issue tracks doing the same in Chromium.) mediawiki.legacy/shared.css forces its own bottom border, causing a double-underline effect. Since only Firefox has added support for the text-decoration-style property, this compatibility rule should be added to shared.css:
@supports (text-decoration: underline dotted) {
abbr[title], .explain[title] {
border-bottom: initial;
text-decoration: underline dotted;
}
}This rule restores the default bottom border (which would be none in most cases) and adds a dotted underline where supported.