Deathfire Touch Keystone on Teemo

No, and thank God for it.

For teemo I would recommend Furvor of battle if you wish to go the ferocity route..

however if you go the Cunning route.. Thunder lords.

Most on-hit effects (like Teemo's E) do not apply on-spell-hit effects (like Deathfire, or Liandry's, or Rylai's...)

I will say this: it works on Swains E, and unless its bugged I've noticed it on teemo's E proc'ing a lot but I'll have to play test it. By the way, if you ever see a swain with deathfire's torch, just buy lots, and lots, and lots, and lots of MR cause you are going to need it.