You don't need a comma. I see your point, though. "Which" might be said to refer to the mountain.
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Anonymousis it clear what the "which" refers to?It depends who you ask! To most native speakers with common sense, it's clear that 'which' refers to the fire.
AnonymousI suppose the best strategy would be to move the adverb phrase, "on top of the mountain", wouldn't it?You could do that, but I don't see any point in attempting to improve on the work of a well-known and respected author.
AnonymousAs a non-native (Polish) speaker, I'm quite obsessed with grammar rules such as those relating to commas and relative clauses.