I think they are just as deadly. Any type of weapon like that is extremely dangerous and also not good for the environment in general. They are extremely toxic and deadly and shouldn't be used at all in any place. They kill and injure so man people each and every year.
Molotov cocktails are not used to destroy things but rather set them ablaze. Their main use is to burn hostiles out of cover or out of vehicles while grenades are use to obliterate targets. Molotovs are also used for their psychological effect as no one wants to be lit on fire. Molotovs can be deadly but grenades are much more lethal
In a Molotov Cocktail attack, the injuries sustained, while they can be life threatening, they are mostly superficial, or at least only 2nd degree burns. When a grenade explodes, everyone in the immediate area is at risk for being caught up in the explosion. The injuries from a grenade attack are massive. Limbs are lost instantly. People bleed to death. More people are affected, usually by a grenade, so it is definitely worse.
I'm sorry but a Molotov cocktail can't do near as much damage as an actual grenade. Maybe back in the day when grenades weren't quite as tactical such as stun grenades and more advanced grenades. Though in today's technologically advanced universe there is no way that a Molotov Cocktail is a bigger threat.
The shear explosive power and shrapnel hurling capability of a grenade renders it far more dangerous than a Molotov Cocktail. The greatest danger of the Molotov is that anybody willing to part with a bit of alcohol, fuel or lighter fluid can quickly construct and hurl them. The damage done on impact, while serious, is nothing like the power of a grenade.