The firmware WG includes, but is not limited to, projects for TianoCore UEFI, U-Boot, Coreboot, TF-M, etc.
The TianoCore mailing list (email@example.com) is not really a place for discussions - it's a place to get code reviewed and approved for merging. Use https://groups.google.com/a/riscv.org/g/fw-exchange for technical collaborations (TBD: need to figure out how to promote this).
China UEFI on RISC-V working group (https://news.sciencenet.cn/htmlnews/2023/3/496231.shtm). The "bridge contact" is Yong Li (Intel). There's no mailing list as the group relies on WeChat.