Making Gecko embeddable is admittedly not on Mozilla's priority list. Certainly not since they abandoned the idea of XUL. https://en.m.wikipedia.org/wiki/Gecko

Making Gecko embeddable is admittedly not on Mozilla's priority list. Certainly not since they abandoned the idea of XUL. https://en.m.wikipedia.org/wiki/Gecko_(software)#Usage https://en.m.wikipedia.org/wiki/Gecko_(software)#Usage Other web browsers using Gecko include Airfox, Waterfox, K-Meleon, Lunascape, Portable Firefox, Conkeror, Classilla, TenFourFox, HP Secure Web Browser, Oxygen and Sylera (for mobile). Other products using Gecko include Nightingale, Instantbird and Google's picture-organization software Picasa (for Linux).