A "firebrand" is a torch, but for Shakespeare, probably not the kind of torch used to light the way for people. In many of Shakespere's other plays the word "torches" is used in stage directions to indicate the presence of torch-bearers who light the way for a group of characters at night. In contrast, the word "firebrand" is used only three times by Shakespeare; in the other two uses, it designates a burning stick used to start a larger fire. So Shakespeare may have in mind something like a lighted match, which produces a flickering light which soon goes out. It is for this reason, I believe, that some editors identify the "firebrand" which Caliban complains about as a will-'o-the-wisp, which is—according to Wikipedia—"is an atmospheric ghost light seen by travellers at night, especially over bogs, swamps or marshes. It resembles a flickering lamp and is said to recede if approached, drawing travellers from the safe paths." Also, folklore said a will-'o-the-wisp was produced by fairies or spirits of nature.
Merida and a will 'o the wisp in the movie Brave, 2012.
