Why should we trust them?
The CA is trusted to do: Determine which certificates are valid.
Firefox is giant. It shouldn't be hard for a malicious party — should one appear someday — to hide some tiny backdoor somewhere in a more-than-a-hundred-megabyte source code tarball.
Verifying GPG signatures of the tarball could prevent some (but not all) issues, but from my observations it's rarely done. And when I've seen it done public key's origin wasn't thoroughly verified, just blindly `gpg --recv-keys`'d from keyserver.