Looks like Mozilla has finally said enough to Adobe, and this might just trigger a cascade of similar actions in other browsers, as well.
Because of the exploit kits available and the recent security vulnerabilities discovered in Adobe Flash, Mozilla therefore has added the current version of Adobe Flash and all previous versions to the browser's blocklist to protect its users (https://addons.mozilla.org/en-US/firefox/blocked/p946) The Flash vulnerability affects all versions of Flash on Windows, Linux and Macintosh systems.
So, Mozilla added the choice option (“Ask to Activate”).
The difference between "ask to activate" and "never activate" is that Flash is not blocked completely in the former state which means that Flash contents can still be accessed in the browser. While that requires an extra click, it ensures that code on websites cannot exploit the vulnerability automatically without user action.
Options to switch the state are not available due to Flash being on the browser's blocklist.
Firefox displays a warning in the browser whenever Flash contents are embedded on a web page. If selected, Flash contents will be loaded and can be used just like before.” – gHacks
So, if you use Firefox and see blocking on web pages, it’s a sign that your version of Adobe Flash Player is outdated and insecure: Update it! Don’t allow a previous version to play as it is insecure.
Source:
http://www.ghacks.net/2015/07/14/mozilla-blocks-all-versions-of-adobe-flash-in-firefox/?_m=3n%2e0038%2e1668%2ehj0ao01hy5%2e1qdz