Why my Gmail goes partially unencrypted whenever I start writing a new message?

Question

When I enter my Gmail account (Ubuntu precise, Firefox with a few addons—AdBlock, WOT, Ghostery, HTTPS Everywhere) the encryption of the page works normally (snapshot 1), and keeps on doing so until I try to write a new message or open a draft. In this case, the connection goes partially unencrypted while keeping the HTTPS status. Information on security of the page also changes (see snapshot 2). Sorry for the snapshots in italian, I guess they're still understandable, I'll add information/translation if needed.

Snapshot 1:

Snapshot 2:

Does anyone have a clue on the possible meaning of this? I’ve searched the web for answers, but to no avail.

I want to add that the same thing also happened with Chromium for Ubuntu and on Chrome.

Answer 1

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.