Starting with Firefox 50, Adobe Flash Player will be hidden from the
navigator.mimeTypes properties when the plug-in has been set to click-to-activate. Since many sites are falling back to HTML5 video when Flash could not be detected, this change is supposed to improve the user experience where the plug-in had to be activated manually. Other plug-ins remain the same at this time.
Update: This change was reverted just before it goes to the Aurora channel (Firefox 49 Developer Edition) due to the broken Flash activation as well as video playback not working on Hulu and Facebook. Mozilla developers are investigating those issues.
Update: The bug described above has been fixed and the patch has been re-landed to Firefox 50. We have updated this document to explain
navigator.mimeTypes is also affected.
Update: This change was disabled during the Aurora development cycle due to a couple of plug-in activation bugs.