The Pale Moon team announced yesterday on the official project forum that the next major version of the browser, Pale Moon 26, will use the new layout and rendering engine Goanna instead of Mozilla's Gecko.
It notes that the impact should be minimal at first, but that there may be some compatibility issues with extensions that use "code paths based on platform versions". Since this is an issue, it is planing on returning a "compatibility version for historical reasons" to make sure these extensions remain compatible with Pale Moon 26.
http://www.ghacks.net/2015/06/22/pale-moon-to-switch-from-gecko-to-goanna-rendering-engine/