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.
Exploitability
AV:NAC:LAu:NImpact
C:PI:PA:P7.5/AV:N/AC:L/Au:N/C:P/I:P/A:P