Pale Moon is a fork of Firefox for Windows and Linux, but it doesn't come down to a silly theming fork or other basic changes. The fundamental differences between upstream Firefox and Pale Moon is that they will not be implementing HTML5 DRM/EME support, they are sticking with the original Firefox interface rather than the new Australis UI, and they will not be accepting sponsored ad pages / tiles.
The Linux builds of Pale Moon right now comes from a PM4Linux project build while Windows appears to be the project's primary focus.
If you're interested in the Pale Moon web-browser, stop by PaleMoon.org. The same developers behind Pale Moon also have FossaMail.org as their fork of the Mozilla Thunderbird mail client.