If the signing subkey was committed, that implies developers have it as a file on their system which I find surprising if true. They should be using hardware like a Yubikey or something. Especially for something this important.

The signing key for Firefox stored on a single hardware yubikey available to a single person?

Could be multiple individuals, each with a different key.

https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing

Multiple hardware devices can have the same key.

People don't need an extra supply-chain failure mode to consider, and CVE proved these dongles are mostly security theater. Likewise, the recent Coinkite user key prediction breach certainly wasn't cool for folks that lost their holdings. =3

Wrong cve and a side channel attack doesn't mean these dongles are useless. It would have stopped the firefox team's ai from commiting their subkey ;)

Storing the secret on a hardware token will most certainly help with not committing into source control.

Most use another build host siloed from the dev staging area, regression tested/audited, and with limited administrative access. =3