CVE 2015-4498
The add-on installation feature in Mozilla Firefox before 40.0.3 and Firefox ESR 38.x before 38.2.1 allows remote attackers to bypass an intended user-confirmation requirement by constructing a crafted data: URL and triggering navigation to an arbitrary http: or https: URL at a certain early point in the installation process.
See the 
CVE page on Mitre.org
for more details.
