Catalysis of the reaction: S-adenosyl-L-methionine + xanthotoxol = S-adenosyl-L-homocysteine + xanthotoxin. Xanthotoxol is also known as 8-hydroxyfuranocoumarin and xanthotoxin as 8-methoxyfuranocoumarin.
furanocoumarin 8-O-methyl-transferase activity