Disclaimer: Work for Mozilla but not on the browser core, thoughts are my own, grain of salt, etc.

My understanding is that on Desktop it's much more difficult than it ideally should be.

With that said we've got a pretty solid story being worked on for mobile via GeckoView [1] and Android Components [2], there's a post on the Mozilla Hacks blog about our use of them in Focus [3] and they're also what is being used for building the "next generation" version of Firefox for Android currently code-named Fenix [4][5].

I wouldn't be surprised if there was an effort to get some of the GeckoView work back onto our desktop platforms.

[1] https://github.com/mozilla/geckoview

[2] https://github.com/mozilla-mobile/android-components

[3] https://hacks.mozilla.org/2018/09/focus-with-geckoview/

[4] https://github.com/mozilla-mobile/fenix

[5] https://play.google.com/apps/testing/org.mozilla.fenix

Yes, it is not exactly easy, but it is not like Chromium Embedded Framework despite being design for embedding is any easier to build.

The truth is the browsers are complicated beasts these days for better or worst and so goes the complexity of building and depending on them.

Speaking as someone who worked on Gecko at Mozilla for many years, that page is years (if not more than a decade) out of date and embedding Gecko is a rather miserable experience.