No idea why you're downvoted: things like [0] are marked as obsolete, GtkMozEmbed et al. are basically dead, and there's no viable alternatives except when you use Android [1]. The only option is to dive deep into Thunderbird's mess of a source code and learn things the hard way, hoping Mozilla will not change things a lot in a road to post-Quantum future.

[0] https://developer.mozilla.org/en-US/docs/Mozilla/Gecko/Embed... [1] https://wiki.mozilla.org/Mobile/GeckoView