Security researcher Bas Venis reported a mechanism where add-ons could be installed from a different source than user expectations. Normally, when a user enters the URL to an add-on directly in the addressbar, warning prompts are bypassed because it is the result of direct user action. He discovered that a data: URL could be manipulated on a loaded page to simulate this direct user input of the add-on's URL, which would result in a bypassing of the install permission prompt. He also reported that in the absence of the permission prompt, it is possible to cause the actual installation prompt to appear above another site's location by causing a page navigation immediately after triggering add-on installation. This could manipulate a user into falsely believing a trusted site (such as addons.mozilla.org) has initiated the installation. This could lead to users installing an add-on from a malicious source.
Red Hat would like to thank the Mozilla project for reporting this issue. Upstream acknowledges Bas Venis as the original reporter.
This issue has been addressed in the following products:
Red Hat Enterprise Linux 7
Red Hat Enterprise Linux 6
Red Hat Enterprise Linux 5
Via RHSA-2015:1693 https://rhn.redhat.com/errata/RHSA-2015-1693.html