Do we know what Toxic thread does?
It's Poisonous Stringshot: -1 Speed and Poison.
He just says that it removes "fire weaks/resists/STAB", nowhere does he say that they become typeless. Since Normal and Fire don't share anything, it's pretty normal that after the move it's that way.
That doesn't say it becomes typeless at all, though. He's just saying that Burn Up removes Fire weaknesses, resistances, and STAB, which is certainly still true if Arcanine becomes a Normal-type after using it.
....Ok, oops. The answer is actually...we don't know. Normal is a strong possibility, but the move needs to be tested.
Also, in an odd bit of non-change, Bellsprout still gets Leech Life through breeding, but it and its Evos aren't compatible with the TM. I though we stopped that nonsense from happening back in Gen 5.
EDIT: I noticed no one mentioned it, so I might as well throw it out there.
Every Starter Line now has Work Up, as do many others who didn't get it previously (due to its lack of TM status in Gen 6). Those would be the Bunnelby line, the Flecthling line, the Skiddo line, the Espurr line, and Hawlucha.
Another EDIT: Jynx, Articuno, Delibird, Regice, Glaceon, Froslass, and Cryogonal get Aurora Veil.