Firefox version 23 introduced blocking of “mixed content,” much like Chrome did last year. Internet Explorer just give a warning unless you have upped the security level of the browser from its defaults. Mix content warnings or blocking occurs when you have a website which uses a security certificate (indicated by the use of “https” in the URL rather than “http” and a it tries to display a page which has content coming from a third party site which does not use a security certificate.
To prevent external websites from be blocked by FireFox or Chrome, you can have the websites open in a new window. This will prevent FireFox and Chrome from blocking the mixed content in Desire2Learn, and let the pages open outside the D2L system.