Firefox 49.0.2 has enabled asynchronous rendering of the Adobe Flash Player plug-in by default, aiming to improve the browser’s stability and performance. It also solves the known issues on 64-bit Windows about Stage 3D support, input method editors (IMEs) and page scrolling.
There are, however, reports of various broken content including video players and games. Mozilla developers are investigating those issues.
Update: Due to considerable regressions, the async plug-in rendering will soon be disabled remotely on the Window 32-bit version of Firefox 49. It has also been disabled on Firefox 50 Beta. Meanwhile on the 64-bit version, it will remain enabled to avoid the issues mentioned above.