I believe it might be still possible with UXP - a hard fork made for Pale Moon project.

Pale Moon is based on a derivative of the Gecko rendering engine (Goanna) and builds on a hard fork of the Mozilla code (mozilla-central) called UXP, a XUL-focused application platform that provides the underpinnings of several XUL applications including Pale Moon. This means that the core rendering functions for Pale Moon may differ from Firefox (and other browsers) and websites may display slightly different in this browser.