Sure it is; says it right on the FB page:
“Pale Moon is an Open Source, Firefox-based web browser.”
Pale Moon is Firefox based through open source. They aren’t working with Firefox, just using the free software for their browser.
“What are the differences with Firefox?
Firefox is created with maximum compatibility in mind, Pale Moon is created with efficiency and speed in mind. These two approaches are pretty much mutually exclusive, and result in some functions being enabled/disabled in one browser but not the other. For extensive detail on the actual differences under the hood, please check the technical details page.
Later versions of Pale Moon also have a number of changes to the user interface and feature set, to provide an as intuitive, predictable, logical and usable user interface as possible for the best user experience. Most of these changes are user-configurable, and can be changed to what you wish. Additional and removed features are, of course, specific to Pale Moon’s build, meaning you may find some different way of working Pale Moon than what you are used to from other browsers.”
“Will Firefox and Pale Moon work together in the future?
Since Mozilla has obviously chosen to follow a different path at the management level, it doesn’t seem likely that Pale Moon and Firefox will ever see a unification or joining of forces.”
http://www.palemoon.org/faq.shtml#What_are_the_differences_with_Firefox