To Firefox, Segoe UI and Segoe UI Light are the same font family. The former is used for normal weight, while the latter is used for lighter weight.

• https://developer.mozilla.org/en-US/docs/CSS/font-weight

1. Install the Stylish add-on.

• https://addons.mozilla.org/firefox/addon/stylish/

2. If you want to ignore fonts specified by web pages and use Segoe UI Light for all elements if possible, paste the following in a new style and save it.

@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document url-prefix("http") {

* {
  font-family: "Segoe UI" !important;
  font-weight: 300 !important;
}

}

3. If you only want to use Segoe UI Light when the web page doesn't specify a particular font (e.g. Wikipedia), use the same style as above, but delete both instances of !important

@Gingerbread_Man

Many thanks indeed for your helpful solution - J P D would be proud of you!

I particularly like the fact that (presumably) this approach could be used to apply other fonts and weights to Firefox.