Features are tracked in bugzilla.
https://bugzilla.mozilla.org/show_bug.cgi?id=501421 covers what you want but nobody is working on it.
If you want to see progress cc yourself on the bug.
If you want it to be implemented then you'll most likely need to either implement it yourself or pay someone to do so as nobody has picked this up in years. If you want implement the feature let us know in the bug and Jonathan and I will try to help you.
Robert.