This change ties into recent concerns and Flash zero-day is the root cause of recent hacking incident in Italy, as well as other recent zero-day incidents.

http://www.pcmag.com/article2/0,2817,2487628,00.asp

Mozilla has blocked all versions of Adobe Flash Player in its Firefox browser after a recent breach left Flash vulnerable to attack.  “When Mozilla becomes aware of add-ons, plugins, or other third-party software that seriously compromises Firefox security, stability, or performance and meets certain criteria, the software may be blocked from general use,” Mozilla said.

Last week, Italy-based Hacking Team was breached and 400GB of sensitive data published online. That data revealed a Flash Player bug, which scammers quickly exploited. Adobe released a patch for that bug, but it was not the only one. So on Sunday, Facebook’s chief security officer, Alex Stamos, suggested Adobe take a more drastic approach (“It is time for Adobe to announce the end-of-life date for Flash and to ask the browsers to set killbits on the same day“)