Historically, it was relatively easy to use Gecko to build other apps including other browsers; what has hard was embedding it inside of something else as just

Historically, it was relatively easy to use Gecko to build other apps including other browsers; what has hard was embedding it inside of something else as just one component among many (i.e. you had to build the whole app using XUL). However, recent-ish (~2015?) changes in Gecko made this harder to do, so many apps that used to do this have switched to the Goanna/UXP fork of Gecko that retains that ease.