Note that this @namespace is used in the userChrome.css file. @namespace url(http://www.mozilla.org

Note that this @namespace is used in the userChrome.css file.

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

For userContent.css you can consider to use this line:

@namespace url("http://www.w3.org/1999/xhtml");

Also note that you can restrict CSS rules to specific domains or URLs.

@-moz-document domain(support.mozilla.org) @-moz-document url-prefix(https://support.mozilla.org/en-US/) @-moz-document url(https://support.mozilla.org/en-US/questions/963378)