My first major contribution since I got hired has landed on the nightlies: Bug 598425. This change lets plugin draw without blocking the browser. This change should improve responsiveness, move more rendering load to the plug-in process and fix a security issue.
Please pay attention to any regression you may encounter with plugin rendering and file them as blocking Bug 598425.
Note: In this context we mean that the drawing is asynchronous from the view point of the browser process. The drawing between the plugin container and the plugin (via NPAPI) is unchanged, for that see the proposed spec NPAPI:AsyncDrawing.