It means that the underlying code for Pale Moon is the same as the code for Firefox; the developers have simply switched off or modified a lot of the less necessary code and switches. Of course, Google's Chrome browser was originally based on Firefox too. The point of switching to Pale Moon, or a similar Firefox derivative, is not to get rid of all Mozilla-originated code; it is to move away from current Mozilla-sourced products and to show Mozilla that stupid decisions have consequences.