Adobe has not supported Android with their Flash Player Plugin since last update on September 10 2013. Besides many sites have long since dropped support of the ancient Flash 11.1.x branch used by Android devices and BlackBerry Playbook.

Besides Mozilla finally dropped support of the old Flash Plugin as of Firefox 56.0 and later for Android. I am not suggesting to use a old version just to try and get Flash loading in Firefox on Android.

Many sites are or have switched to using HTML5 players for video, audio and games to be more cross platform compared to the more limited Flash Plugin platform use.

Besides, in early 2020, Flash support will be completely removed from consumer versions of (desktop) Firefox. The Firefox Extended Support Release (ESR) (desktop) will continue to support for Flash until the end of 2020. Other web browsers have dropped or will be dropping Flash early next year also.

Adobe is in process of discontinuing their Flash Player Plugins with End of Life in late 2020.

https://developer.mozilla.org/en-US/docs/Plugins/Roadmap

At that point any websites still only using Flash for video, audio, games will be outdated or behind the times.