Before everyone gets up in arms about Mozilla not working on this: As I wrote the last time this came up, deterministic builds are a nice thing, but they're only a small piece of the puzzle of protecting users from the state-sponsored malicious actors. Indeed, it seems to me that messing with builds would be one of the more difficult ways for the NSA to pwn Firefox users.
https://news.ycombinator.com/item?id=7045605