Mozilla plans to disable Adobe Flash in Firefox 69 by default according to an updated bug listing on the organization's bug-tracking website.
Adobe Flash Player is the last NPAPI plugin that Mozilla Firefox supports; support for other NPAPI plugins like Microsoft Silverlight or Java was removed in Firefox 52. Firefox users could switch to Firefox ESR to continue using NPAPI plugins at the time. Google dropped support for NPAPI plugins in 2015 in Chrome.
Mozilla, Google, and other browser makers announced that Flash was on its way out, and Adobe decided to retire Flash in 2020..
Mozilla's Flash retiring timeline lists two Flash related events for 2019:
Disabling means that Flash cannot be used anymore by default unless activated again by the user. Firefox won't prompt users anymore to enable Flash when sites require it, but it will be possible to enable Flash in the browser.
The next steps in the Flash deprecation happen in 2020 and 2021. Flash support is removed completely from all Firefox versions except for Firefox ESR in 2020. Firefox ESR will continue to support Flash until the end of 2020.
When Adobe stops the release of security updates for Flash, all Firefox versions won't load the plugin anymore.
The real downside to this, of course, is auto-playing embedded HTML5 advertisements replacing Flash based ones. More secure yes, but far less easily blocked by browsers without extensions. Another problem is that HTML5 video, like you find on news sites, are also set to autoplay and cannot be blocked, as they do not depend on a plugin or extension, without blocking that domain from loading with NoScript or similar. This wouldn't be a major problem except in the USA, a country with the worst internet in the developed world, unlimited internet is simply not an option for tens of millions of people (not counting the "unlimited" which are just "unlimited until you use XX, then you get reduced speeds), and those megabytes add up...
And I speak as one of those people who, at home, get 20GB per month, 666MB of data per day, before speeds are reduced to 1mbps, and in 2019 terms, that's slower than 56k.