For my greatest trick, I will now try to predict the Pokemon future
First of all, the grass typing is already a disadvantage. Thankfully, the Fire-Typing would cancel out 3 of the 5 weaknesses that grass has, but adds a rock weakness to it. It would have 2 regular resistances and 1 double-resistance. The average stats for Grass-types rank an average of about 11 out of 15 in every stat. Fire types have the 3rd strongest Special Attack but aren't defensive. In Attack, Special Defense, and Speed, the fire type is 7th out of 15 types. So predictably, a Grass-Fire typed Pokemon would most likely have a good Special Attack and Speed (basically, a Special Sweeper). It would most likely have Grass and Fire type moves, along with a possibility of Normal-type moves, Hidden Power, and some TMs.
It would most likely be a Special Sweeper. Because it most likely will not have the best defense, it might be in UU due to its possible low defense. It would most likely be good offensively, and okay Specially Defensive.
Poison only has 2 weaknesses, and Steel has 3. The Poison typing cancels out 1 weaknesses of Steel (Fighting), and the Steel cancels out one of Poison's weakness (Psychic). At the end, it would have 1 regular weakness (fire), and 1 double weakness (ground). It would have 1 immunity (poison), 6 regular resistances, and 3 double resistances. Due to Steel types having the best Defense, good Attack, and Good Special Defense, immunity to Poison/Toxic, and the numerous resistances, this Pokeon would most likely be a tank (a Pokemon that can take hits and return them). However, due to its 4x weakness to Ground, it might use air baloon more (as Astronautical stated in the comments). It might even get Levitate, though it is extremely unlikely. It would most likely get Poison and Steel moves, along with a possible Normal-typed moves, Hidden Power, and maybe even Ground and Rock moves.
It would most likely be a Physical tank, altough it could also be a Special tank. It would most likely be in OU, despite the 4x weakness to Ground (look at Heatran). It would most likely be defensive and offensive. Basically, it would most likely be the Poison-Steel version of Heatran.
I hope this helped!