Oh, sure, you could absolutely make a case for your character accidentally stumbling on the right answer simply because fire is a good weapon, and a good roleplayer could use that to their advantage to metagame a bit more acceptably, but there's a difference between that and just automatically grabbing fire stuff because you the player know it's good against trolls.

Trolls only come out at night. As a city dweller who doesn't go out at night you may only know children's stories about how if you go out at night you may be eaten by a troll. If your monster lore roll is good enough, maybe you heard the story where the guard killed the troll with burning pitch when the child escaped back though the gate and the troll got too close to the city wall

sucking venom

That will kill you if you're bitten by an Australian snake. For Australian snakes you need a compressive bandage long enough to bandage the entire limb that got bitten, and you need to bandage it tight, starting close to the trunk and working towards the foot (or hand if you were stupid enough to put your hand near a snake), then all the way back up. Two layers of bandage. Then the casualty needs to relax as well as they can while their companion goes to the nearest phone signal and calls for emergency transport.

If you want to avoid all that hassle, make sure you see the snake so you can stop 3 or so metres away and it will move away - it doesn't want to mess with you, you're big and scary. It'll only bite you if you corner it or step on it, or get too close for it to feel like it can safely escape

But that advice might get you killed in India, their snakes might attack you if you stop near them, perhaps you should run away. I'm not sure though, I don't live near those snakes

And while I do know is about land snakes in the southern Australian states. I don't know much about sea snakes up north, nor the tropical snakes, aside from the bandaging which is snakebite first aid all over Australia

Ed. Correcting swipe type errors

Those are both great examples of why a character might try to throw anti-troll paste (tar) at trolls.

Funny that you use those examples, since they're both wrong. If anything, we should be running at the trolls with stinky socks because someone wanted to sound smart at a bar once and made up a factoid that spread like wildfire among the common folk.