Their plans are readily available on the site I supplied above. If you're really interested, you'll go read about it. If you're not really interested, I can summarize, and say that they are cherry picking.
Let me make this a little plainer. Pale Moon took the Ghekko engine at - uhhhhhh, version 27 I think it was - and "froze" it. That is what they are working with. Yes, they have broken compatibility with some Firefox things. No, they don't intend to implement all the "improvements" that Firefox builds in the future.
Now, which part of "fork" did you fail to understand? It is not Firefox anymore. Pale Moon is somewhat backward compatible with old versions of Firefox, and addons that worked on old Firefox will either work, or can be made to work on Pale Moon. Newer addons grow into big problems, quickly.
Pale Moon has no intention of remaining compatible with Firefox, and as time goes on, it should look less and less like Firefox.
Now, please don't make me try to justify Pale Moon anymore. I'm just a user who has been moderately well impressed with it. I'm not a developer, an investor, a fanboy, or anything else. Just a user, who is pleased with the dramatic difference in resource consumption on an aging computer. Pale Moon is treating me well.