CPOWs are wonderfully magical things that allow us to synchronously manipulate objects in another process just as if they were running in the parent process. CPOWs have helped ease add-on compatibility problems and have also made it possible to adapt parts of the front-end to work with e10s without too much difficulty. The problem with CPOWs, however, is speed. By their very nature, CPOWs block the main browser process while they communicate with the content process that houses the object we're talking to. That's bad for performance. Inefficient CPOW usage can result in a ton of synchronous traffic resulting in noticeable jank or lag in the browser - something that e10s is supposed to be fighting. We should try to reduce to a bare minimum how much core Firefox code depends upon CPOWs. This is a metabug to track that effort.