Firefox will not ship HTML Imports in its current form. See this status update for more information. Until there is concencus on the standard or alternative mechanisms are worked out, you can use a polyfill such as Google's
HTML Imports is intended to be the packaging mechanism for Web Components, but you can also use HTML Imports by itself.
<link>tag in an HTML document like this:
<link rel="import" href="myfile.html">
import link type is new.
|HTML Imports||Working Draft|