Avoid using deprecated RegExp.$1

src/browser.js

@@ -125,14 +125,16 @@ function useColors() {
 		return false;
 	}
 
+	let m;
+
 	// Is webkit? http://stackoverflow.com/a/16459606/376773
 	// document is undefined in react-native: https://github.com/facebook/react-native/pull/1632
 	return (typeof document !== 'undefined' && document.documentElement && document.documentElement.style && document.documentElement.style.WebkitAppearance) ||
 		// Is firebug? http://stackoverflow.com/a/398120/376773
 		(typeof window !== 'undefined' && window.console && (window.console.firebug || (window.console.exception && window.console.table))) ||
 		// Is firefox >= v31?
 		// https://developer.mozilla.org/en-US/docs/Tools/Web_Console#Styling_messages
-		(typeof navigator !== 'undefined' && navigator.userAgent && navigator.userAgent.toLowerCase().match(/firefox\/(\d+)/) && parseInt(RegExp.$1, 10) >= 31) ||
+		(typeof navigator !== 'undefined' && navigator.userAgent && (m = navigator.userAgent.toLowerCase().match(/firefox\/(\d+)/)) && parseInt(m[1], 10) >= 31) ||
 		// Double check webkit in userAgent just in case we are in a worker
 		(typeof navigator !== 'undefined' && navigator.userAgent && navigator.userAgent.toLowerCase().match(/applewebkit\/(\d+)/));
 }