Digging deeper I found the problem is with https://github.com/gfx-rs/wgpu which Firefox includes in the distfile. Building wgpu v0.9.0 fails with Rust 1.64.0, v0.10.0 has been re-written, but I am not sure it is suitable for Firefox 91.
Firefox 92.0.1 is the last version that includes the problematic code. 93.0 has the newer wgpu. Maybe that is suitable for us? Nia?