For now, only nightly users will get this change. Riding the trains depends on bugs 913503 and 904890, and general stability. We wanted to land this early to get some extra testing time and because without being tested, it has been rotting super quickly. I will arrange for a branch to keep testing main thread composition asap.
One known issue is windowed plugins lagging during scrolling (913503), so please ignore that (for now) if you observe it.
OMTC can be disabled by setting the 'layers.offmainthreadcomposition.enabled' pref to false. If there are more problems than we can fix relatively quickly, we can do this for all users very easily.
This is the culmination of months of work, so it is really exciting to see it finally land. I fully expect, however, to have to turn it off due to a tidal wave of unforeseen bugs, but such is life.