A gray padlock means that Firefox connected securely, but that the site didn't get a more expensive Extended Validation SSL certificate. In order to get an EVSSL certificate, which shows a green padlock in Firefox, the company which operates the website has to verify additional details to the certificate issuer. Both Firefox and Chrome normally should display the same information even if the lock icon looks different. If the site has an EVSSL certificate, the company name will be displayed next to the padlock. If you get the full EVSSL display in one browser but not another, that would be suspicious. That could indicate that in one browser an intermediary is intercepting your connection (securely, but you would want to make sure you trust them to be in the mix). But if you get a plain lock in all browsers, the difference in color probably is just a stylistic difference. Probably.