Firefox will begin retiring Adobe Flash on August 2 with the release of Firefox 48. In 2017, probably with Firefox 53, Flash plug-ins will require the user to actively click-to-play.

In Firefox 48, Mozilla will enable a new Firefox plug-in blocklist by default. Initially the blocklist will be small, mostly containing URLs of Flash SWF files that have been identified by Mozilla as supercookies (i.e. cookies that are very hard to shake off) or fingerprinting files (i.e. they scan your system and create a unique fingerprint, again usually for tracking purposes).

The Github repo explains the criteria for adding new SWFs to the blocklist:

Blocking the content will not be noticeable to the Firefox user

It is possible to reimplement the basic functionality of the content in HTML without Flash

The Mozilla blog post introducing this blocklist only mentions supercookies and fingerprinting files in passing, though. Rather, Mozilla explains this move as the beginning of the end for Flash: "... Plug-ins often introduce stability, performance, and security issues for browsers. This is not a trade-off users should have to accept." Thus, Mozilla will gradually add Flash elements that are "invisible to users" to the blocklist, which is "expected to reduce Flash crashes and hangs by up to 10 percent."

Then, in 2017 (probably Firefox 53 in May), Flash content will be entirely click-to-play: so, you'll still be able to play Flash games and watch Flash videos, but all of those Flash ads and hidden Flash elements will be completely blocked. (This is functionally very similar to how Chrome has handled Flash elements since September 2015.)

Mozilla hasn't directly stated that Flash will be completely retired after the click-to-play change, though that is clearly the ultimate end goal. With Firefox 52, released in March next year, every NPAPI plug-in (Java, Silverlight, etc.) will be disabled... except for Flash. Click-to-play is Mozilla's way of not-so-gently pushing the last few holdouts away from Flash before the guillotine falls.

Firefox 48, due out on August 2, will also be the first time that Electrolysis (splitting Firefox into a content process and a UI process) will be rolled out to the masses. Mozilla's attempts to win back developers and users continues!