As part of the ongoing NPAPI plug-in deprecation in Firefox, Flash content is now defaulted to click-to-activate in Firefox Nightly, after some advanced standard technologies including
SharedArrayBuffer, WebAssembly and WebGL 2.0 have been implemented and shipped to public.
The current plan is to roll out this change gradually to the Beta and Release channel user base of Firefox 55. Web publishers must have a clear plan to move away from any Flash dependency to offer the best possible user experience.
Update: This change has been rolled out to all Firefox 55 users as of September 20.