Gmail – How to encrypt email messages and attachments without using an extension in Gmail

Here is a Greasemonkey script for example: http://www.langenhoven.com/code/emailencrypt/gmailencrypt.php

Also try to google for Gmail + PGP!

Hope this helps.

That's a "Mixed Content" problem; meaning that, although the main page was served securely via HTTPS, some of the resources used by the web page were retrieved using insecure HTTP.

The Mozilla Blog Mixed Content Blocking Enabled in Firefox 23! has much more detail about this issue as well as describing how to configure Firefox to block mixed content:

(2) To block Mixed Passive Content, open a window or tab in Firefox and enter about:config. You will get to a page that asks you to promise to be careful. Promise you will be, and then change the value of security.mixed_content.block_display_content to true by double clicking it.

(3) In Firefox 23+, Mixed Active Content is blocked by default. If you are using a Firefox version between 18 and 22, you can block Mixed Active Content by opening a window or tab in Firefox and enter about:config. You will get to a page that asks you to promise to be careful. Promise you will be, and then change the value of security.mixed_content.block_active_content to true by double clicking it.

To find out precisely which content is getting sent via HTTP, the Net panel in the Firebug extension for Firefox will show all of the network requests being made from a web page.