fix CSS blur filter due to performance issue in Firefox

_sass/layout/_menu.scss

@@ -146,8 +146,15 @@
 	}
 
 	body.is-menu-visible {
-		#wrapper {
-			@include vendor('filter', 'blur(0.5em)');
+	    #wrapper {
+		-webkit-backface-visibility: hidden;
+		-webkit-perspective: 1000;
+		-webkit-transform: translate3d(0,0,0);
+		-webkit-transform: translateZ(0);
+		backface-visibility: hidden;
+		perspective: 1000;
+		transform: translate3d(0,0,0);
+		transform: translateZ(0);
 		}
 
 		#menu {
@@ -161,4 +168,4 @@
 				visibility: visible;
 			}
 		}
-	}
+	}