The fire burns merrily, the smoke swirling and stinging your eyes. On a successful perception check (DC 3) you notice that almost none of the smoke is entering the mine, because that's not how air currents work! If you're lucky, the goblins sleep through it. If you're unlucky, the guard smells the fire and wakes the others to rush you. If you're very unlucky, the goblin hidden amongst the rocks above the entrance sees you and starts taking shots at you with their bow, whilst also alerting their companions in the tunnel. Roll for initiative.

Congratulations, you now have one massive wave based fight at the mine entrance instead of multiple smaller encounters inside. If you succeed on an X check, some of the weaker enemies have penalties due to the smoke and fire.