The Mozilla Toolkit is a set of APIs, built on top of Gecko, which provide advanced services to XUL applications. These services include Profile Management, Chrome Registration, Browsing History, Extension and Theme Management, Application Update Service, and Safe Mode. (More info)
This service should only be used internally by Places, so it's worth removing it from PlacesUtils and manually instantiating the service only where needed to reduce confusion.
Consumers should instead keep using PlacesUtils.history.