Fire Bombs were more often than not make-shift weapons consisiting a glass or clay jar filled with a flamable liquid such as turpentine or whale oil or other spirits. A rag soaked in the same liquid and then fed through the neck of bottle to act as a wick or fuse. This fuse would be lit just before the bomb is thrown. Upon impact with the ship's deck or other hard surface the glass would break and the burning fuse would fall into the liquid and cause it to burst into flames.
As a general rule, fire bombs were not used much. Most ships kept buckets of sand handy in the event of a fire. If a fire broke out on a deck sand would often be used to smother it. Also to make it less likely for the ship to burn (despite all the pine tar used in the cracks), a ship's deck was often soaked with water, especially before a battle.