Although it isn’t actually released yet, the plan is simply for an initial multiprocess setting to be made at install or upgrade based on whether you have addons or a11y. After that you are on your own. You can enable or disable multiprocess or change anything else such as installing addons.
Many addons work well in multiprocess mode, many are slower, and quite a few don’t work or cause random crashes and hangs. Many accessibility features operate very poorly across multiple processes. Firefox itself is largely working in multiprocess mode, but with some performance issues which will hopefully be fixed up by the time 46 comes out. It is likely that there will be other factors feeding into whether e10s mode is turned on and how many content processes will be configured, for example whether the hardware even has multiple cores.