HTML Imports still disabled by default in Firefox

Although work on HTML Imports is completed in Firefox, the feature is still disabled by default (verified in Firefox Beta 35.0 and Firefox Nightly 37.0a1 (2014-12-05) using http://jsbin.com/wizecamiji/1/quiet).

Reverts 2f872bc0a2c2a4fb178a010e390ae437b5ebc981