Today Firefox announced support for preload will be added to today’s build of Firefox.

<link rel=preload> support will land in today's build https://t.co/Gk6nidJa2e

— Firefox Nightly (@FirefoxNightly) July 11, 2017

Using preload links to declaratively specify which resources the user agent must fetch early to improve page performance. 

According to https://caniuse.com/#search=preload preload is supported in Chrome, Opera and Android browser (Firefox has yet to be added)

Early fetch of critical resources example:

<link rel="preload" href="/assets/font.woff2" as="font" type="font/woff2">
<link rel="preload" href="/style/other.css" as="style">
<link rel="preload" href="//example.com/resource" as="fetch" crossorigin>
<link rel="preload" href="https://fonts.example.com/font.woff2" as="font" crossorigin type="font/woff2">