Recently, to improve Firefox browser stability and performance, Mozilla has turned on Asynchronous Plugin rendering by using a system add-on. Behind the scenes, the system add-on flips the dom.ipc.plugins.asyncdrawing.enabled preference value to true.
Due to regressions, the browser vendor has now decided to disable async rendering by default in Firefox 49.0.2 on Windows 32-bit using a system add-on, however, the feature remains enabled on Windows 64-bit, because this could cause more regressions in page scrolling and IME support.
If you don’t want Mozilla to ship the system add-on for you to turn off async rendering in Firefox 49, then visit about:config and change the below preference to false. FYI, Async drawing has been already disabled on Firefox 50 beta.