The gray ! warning triangle can indicate a few different things:

• There are images or other lower-risk content in the page that was retrieve over HTTP instead of HTTPS

• The site uses an encryption cipher that Firefox no longer considers to provide sufficient security (such as RC4 ciphers)

To investigate the first issue, you can try using Firefox's browser console. You can open the console using Ctrl+Shift+j (or the Developer menu). After a moment some text will appear. Click the Clear button at the top and then in the filter box at the right type mix and leave this window open. Reload the page that is showing the triangle, then return to the console to see whether any matching error lines show up (mentioning mixed content). Anything?

To investigate the second issue, you can click the triangle, then More Information, to load the Security panel of the Page Info dialog. There should be a paragraph at the bottom which describes the level of security offered by the server. It may be gibberish to you, so feel free to ask about it. I don't think it's possible to copy/paste it, unfortunately, but you can grab a screenshot (e.g., Alt+PrintScreen, then paste into an image editor like Paint). Please cut or blur any sensitive information in the image before attaching it to a reply.

Let me know what you find so we can rule out any mischief.

By the way, this sounds similar to another user's experience on PayPal recently, where the company receiving the payment was embedding its logo into the page using HTTP instead of HTTPS: Firefox Site Identity button.

See also:

• https://developer.mozilla.org/Security/MixedContent
• https://support.mozilla.org/kb/how-does-content-isnt-secure-affect-my-safety