I just watched the newest episode, and when Jet strikes swords with Zuko, I heard a spark sound and the room got brighter. Sparks and light are components of firebending, and two firebenders with anger issues could be emitting it in??enough quantities to be noticeable, so Jet has to be a firebender! Now, I know his parents were from the Earth Kingdom, but what if they were only adoptive parents? A firebending woman who had been exiled or ran away could??have died in the Earth Kingdom in either childbirth, or soon after, so there would be no recellection on Jet's part, nor would he have been trained firebending.
There's only one woman I know that could possibly fit that category: ZUKO'S MOM! If this is true, then not only is Jet a firebender, but he's royalty!
Again, this is just speculation, but it seems possible.
Edited 2 total times.