Requires a bottle filled with an alcohol, something along the lines of firebreather instead of simple ale. A strip of cloth, a thief could tear/cut items like tunics or wool leggings and the like. The skill would essentially allow the theif to light the strip on fire and then throw it into an adjoining room either causing an explosion or causing the room to catch fire.
Depending on the strength of the thief they may be able to throw larger objects such as barrels. Increased item weight would cause a more likely fail. Should a thief fail the toss there is a chance the container will spill and suffocate the fire, or the thief would catch himself on fire, or the thief drops the container and it explodes in the room causing damage to the thief. If there are things or possibilities I left out I do apologize.

Oh and on edit, so that this skill isn't abused against senitinel mobs, they could be resistent or immune to the effects.

I liek the basic principle of thieves creating throwable items (most specifically flasks of some content). I think it coul deven be expanded to more generically with the following types of contents:

flammable oil,
acid,
poisonous gas
etc.

yeah i would like to brew some molotovs