If you weren't concerned before, don't be concerned now: the risk hasn't changed, now you are just being notified it exists. Google's Chrome has been flagging insecure sites since January but less obtrusively by just displaying an icon showing sites are not using HTTPS (HTTP with SSL/TLS) encryption. An industry push for using that encryption is changing tone from "not recommended" to "unacceptable" for sites that don't comply. Firefox is just going along with that push by explicitly informing browser users of unencrypted connections when they are asked for data input.
You would think this site's caretakers, who specialize in optimizing design, would have include using HTTPS in the site conversion after purchase.