As Jake Archibald says, they are starting to land! The support landscape is already:
Chrome Canary 60 – behind the Experimental Web Platform flag in chrome:flags.
Firefox 54 – behind the dom.moduleScripts.enabled setting in about:config.
There are plenty of weird gotchas to be aware of, like minor syntax things that are intentionally not supported, and the order of script execution.
We covered Stefan Judis’s …
ECMAScript Modules in Browsers is a post from CSS-Tricks