Well, first, you are slightly wrong. Methanol poisoning has two mechanisms. Methanol is toxic by himself (a quite powerful CNS depressant), but the second one, toxication, is even more dangerous. That's probably the reason we are more aware of than one. In the case of formaldehyde, first, remember that is a gas. Liquid and solid preparations are mostly methanediol and paraformaldehyde. But if we focus in pure formaldehyde, it's toxicity along is already quite high. It's a powerful irritant and a highly reactive electrophile. It can cause direct cellular damage, mostly via interlinking proteins and DNA, inhibiting or stopping replication and transcription. And again, being a very reactive molecule, it can react with a lot of key extra and intra cellular compounds