What does the message look like? Is it a full page indicating a problem with the site, or a message on the downloads panel? For comparison, see: How does built-in Phishing and Malware Protection work?

I don't think Mozilla monitors the "companion" or "native application" that add-on developers distribute. If your security software says the application is not safe, then perhaps you should not install it.

See : https://www.downloadhelper.net/install-coapp

And scroll down to :

'How safe is it to install and run the companion app  ? ' and 'How hard is it to install and run the companion app  ? '