Firefox will not ship HTML Imports. See this Hacks blog post for more information. You can still use HTML Imports in Firefox by enabling the dom.webcomponents.enabled flag. If you don't want to enable the flag, you can use a polyfill such as Google's webcomponents.js.
HTML Imports is intended to be the packaging mechanism for Web Components, but you can also use HTML Imports by itself.
You import an HTML file by using a
<link> tag in an HTML document like this: <link rel="import" href="myfile.html">
The import link type is new.
Specification
| Specification | Status | Comment |
|---|---|---|
| HTML Imports | Working Draft |