fonts: add fira sans (#41295)

README

@@ -273,6 +273,15 @@ copyright notices:
   #
   # https://www.fontsquirrel.com/license/yantramanav
 
+* Fira Sans
+  # Copyright (c) 2014, Mozilla Foundation https://mozilla.org/
+  # with Reserved Font Name Fira Sans.
+  # Copyright (c) 2014, Telefonica S.A.
+  #
+  # This Font Software is licensed under the SIL Open Font License, Version 1.1.
+  #
+  # https://www.fontsquirrel.com/license/fira-sans
+
 ~~~~
 
 Some theme variant files have their own specific licenses; refer to

static/includes/_font-firasans.scss

@@ -0,0 +1,26 @@
+@mixin firasans-font($type, $weight) {
+	@font-face {
+		font-family: 'Fira Sans';
+		src: url('/static/fonts/FiraSans/firasans-#{$type}.woff2') format('woff2'),
+			 url('/static/fonts/FiraSans/firasans-#{$type}.woff') format('woff');
+		font-weight: $weight;
+		font-style: normal;
+	}
+	@font-face {
+		font-family: 'Fira Sans';
+		src: url('/static/fonts/FiraSans/firasans-#{$type}italic.woff2') format('woff2'),
+			 url('/static/fonts/FiraSans/firasans-#{$type}italic.woff') format('woff');
+		font-weight: $weight;
+		font-style: italic;
+	}
+}
+
+@include firasans-font('thin', 100);
+@include firasans-font('extralight', 200);
+@include firasans-font('light', 300);
+@include firasans-font('regular', 400);
+@include firasans-font('medium', 500);
+@include firasans-font('semibold', 600);
+@include firasans-font('bold', 700);
+@include firasans-font('extrabold', 800);
+@include firasans-font('black', 900);