As far as I know there is some scope for community involvement. @gsvelto can confirm what’s happening on the Mozilla side, I think there are some internal hurdles to pass before the project branch can be posted publicly and people can start to contribute.
On the KaiOS side @fabrice has posted links to the KaiOS version of gecko in the Matrix chat channel:
A b2g-desktop build can be made from that. Gonk support not included yet, but it will only support Android 10 base and later.
Please join us in the Matrix channel for the most up to date discussion, although I think some topics would be better discussed here in Discourse to keep them focussed and more accessible to those who aren’t continuously in Matrix.