The setting that Severine mentioned controls the number of content processes, which is the number of processes that Firefox uses for its tabs. It will still ha

The setting that Severine mentioned controls the number of content processes, which is the number of processes that Firefox uses for its tabs. It will still have another process that controls all the UI and all those browser tab processes, a process for sandboxing extensions/plugins and then even more processes for miscellaneous things, for example I think asynchronous scrolling has another process and when doing performance improvements, they often just stuck long operations into a separate process.