Very important that you develop complete confidence that there isn't anything wrong with your product. It's not uncommon, in fact it's very common, for compromise kits for websites to take measures to avoid detection. A common one is only serving the malicious content when a specific referrer is present (I've seen this be Yahoo Search in the case of compromised Drupal installations multiple times, not really sure why…
Should they then not just reply with "You're on the list because of the malware payload at "?
should? probably. But I also get, from a capitalist perspective, why they don't: they probably get enough "we swear our website is actually fine" tickets each day that they would need a sizable dedicated team to offer that kind of assistance. I don't think any of the browser vendors, Google and Mozilla included, will go to any real effort to help the reporter. At least I haven't seen them do so. I think they take the view, and I don't totally blame them, that securing your website is your problem, and they aren't going to offer security consulting for free.