Similar to Google's efforts to demote Flash in Chrome while still supporting it, Mozilla said it will block "certain Flash content that is not essential to the user experience" from August.
Then, at some point in 2017, it will turn Flash off by default in Firefox and require users to click in order activate the plugin. It will also block invisible Flash content.
Websites that use Flash or Silverlight for video or games should plan for HTML technologies "as soon as possible", Mozilla said. These plugins are increasingly becoming redundant as more capabilities are brought by HTML5, including audio and video playback, streaming capabilities, 2D and 3D graphics, and access to components such as the microphone and camera.
Reminding that Google said Flash in Chrome would be disabled for all websites except the top 10 globally that still rely on Flash Player. That move doesn't affect video on sites such Facebook and YouTube, which have already moved to HTML5. Even Adobe, though committed to still developing Flash Player, is moving toward HTML5.
source and more info at ZDNet
