If you look at the firefox page in Koji, it looks like the F31 builds of fireox had been failing lately - https://koji.fedoraproject.org/koji/packageinfo?packageID=37
Last successful build was firefox-68.0-5.fc31 on July 17. And that was included in the most recent Rawhide/F31 ostree compose:
$ sudo ostree pull --commit-metadata-only --depth=1 fedora:fedora/rawhide/x86_64/silverblue
GPG: Verification enabled, found 1 signature:
Signature made Sun 21 Jul 2019 02:20:42 AM EDT using RSA key ID 50CB390B3C3359C4
Good signature from "Fedora <fedora-31-primary@fedoraproject.org>"
Receiving metadata objects: 3/(estimating) 22.6 kB/s 67.9 kB
GPG: Verification enabled, found 1 signature:
Signature made Sat 20 Jul 2019 12:56:11 PM EDT using RSA key ID 50CB390B3C3359C4
Good signature from "Fedora <fedora-31-primary@fedoraproject.org>"
4 metadata, 0 content objects fetched; 100 KiB transferred in 4 seconds
$ ostree rev-parse fedora:fedora/rawhide/x86_64/silverblue
51b5f5287a15f02e7ba4ed316ad8ee66c0f41aa1a6e767432b83dafdab799e1a
$ rpm-ostree db list 51b5f5287a15f02e7ba4ed316ad8ee66c0f41aa1a6e767432b83dafdab799e1a | grep firefox
firefox-68.0-5.fc31.x86_64