Firefox will not ship HTML Imports in its current form. See this status update for more information. Until there is a consensus 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|