[Steam] (Alpha/Beta) Facepunch Prototypes

Same in firefox.

The owner of prototypes.facepunch.com has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.

it's because OP link has https while the site not.
You can search for Facepunch Prototypes (as I did) and it most probably be the first result.
Or you can click here: http://prototypes.facepunch.com/ :)

In fact, the problem is that by going to the secure version, you're using the hosting's global SSL certificate (*.azurewebsites.net) which doesn't match the address of prototypes.facepunch.com. Not a big issue, just accept the SSL certificate temporarily and go on.

BTW, doing so is more secure to access the non-secured (http://) site. Even if the certificate is not valid for the site, the communications are encrypted, while using http protocol they're not. The worst that could happen if you use the HTTP protocol is that someone could be able to snoop your Steam key (the Steam login is done securely on Steam's servers), but nothing more. And that can't happen (or at least it becomes way more difficult) if you're using the secure connection (even with the wrong SSL certificate).

Because checking from an external website if you own the DLCs and subs is a pain in the ass (like it happens here on Stamgifts for the giveaways, for example) :)