Came across this gHacks article via Grand Stream Dreams. Starting with Pale Moon 26 the rendering engine is going to move from Gecko to Goanna. From what I gather, the change is more for legal reasons (Gecko is registered trademark licensed to the Mozilla Foundation by Netscape/AOL) and also to fix compatibility issues with websites detecting the wrong version of Gecko because of the numbering differences with Pale Moon and Firefox. There is really not going to much changes under the hood so to say.
It is going to be a fork off of Gecko much like Chromium (Open Source Project based off of Chrome) using Blink, a fork of the WebKit engine. I think this could be a way for Pale Moon to disassociate itself with the bloated mess Firefox has become now. However, Pale Moon has always used a custom build of Gecko (which does not include the bloat and the drastic UI changes).
I find it interesting though that they are citing the legal ramifications for using the Gecko name as one of the reasons for this change. Makes one wonder if making this change now is to avoid possible legal complications in the future with The Pale Moon Project and Mozilla. Could Mozilla one day decide that if you are going to say your browser uses Gecko, then it must contain certain elements/features of Gecko (which may include some that are not desirable for the Pale Moon Project)? I don’t see Mozilla doing this, but then a lot has changed with Mozilla over the past few years…so anything is possible.