This document provides an overall look at the architecture of the profiler, and how all the various pieces fit together.
Please note, this is the diagram for the new recording panel in DevTools that is off by default. This is the path forward for integrating profiler.firefox.com into DevTools. It can be enabled using devtools.performance.new-panel-enabled. There is no architecture doc for the current Firefox DevTools panel.
Remote profiling with DevTools is very similar to the above diagram, but involves slightly different transport messaging mechanisms in order to target remote devices.
Firefox can configure the Gecko Profiler using about:profiling as well as the menu button that can be enabled via the profiler.firefox.com homepage. These UIs interact directly with the nsIProfiler interfaces via Services.profiler. The documentation for those components live in mozilla central.