Mozilla is developing a feature in Firefox that would require some user interaction in order for Flash ads, Java scripts and other content that uses plugins to play. In addition to easing system slowdowns, the opt-in for Web plugins is expected to reduce threats posed by exploiting security vulnerabilities in plugins, including zero-day attacks. The Firefox Nightly channel has an early rendition of the click to play permission model and instructions on how to load it for review. Currently there are Firefox add-ons that do something similar, such as the NoScript extension.
Source: https://threatpost.com/mozilla-weighing-opt-requirement-web-plugins-041212/76441/