Firefox 40 has enabled asynchronous plug-in initialization for a better performance, as Aaron Klotz explains in his blog post. As the side effect, specific plug-in content, such as Flash videos or games including FarmVille 2, may not be loaded properly depending on the code design. If you change the
dom.ipc.plugins.asyncInit preference value to
about:config and your issue can no longer be reproduced, it is caused by the async initialization. Developers are encouraged to file a bug while consulting the corresponding plug-in vendor to work around the issue.
Update: Due to considerable breakages, this change was backed out via the Firefox hotfix add-on updated on as well as Firefox 40.0.3 shipped on . A radical solution to the issue has been created and implemented to Firefox 41.
Update: In order to avoid conflict with the hotfix add-on, Firefox 41 has renamed the preference to
dom.ipc.plugins.asyncInit.enabled while the default value (
true) remains the same.