Earlier quoted context omitted.
They have Nightly branding but they aren't nightlies.
When I download the Unbranded/"Nightly" build, it says it's v107.0.1, versus Stable is v109.0. You seemed to be of the impression the declared version string was incorrect, so I dug a bit deeper. The security fixes mentioned in the changelog for v109, such as CVE-2023-23601 [1] are not present in the latest Unbranded/"Nightly" release. So basically Mozilla telling developers: want to be able to run your own code? Fin…
• https://wiki.mozilla.org/Add-ons/Extension_Signing#Unbranded... (click “release”)
• https://treeherder.mozilla.org/jobs?repo=mozilla-release&sea... (click the date of the most recent complete build)
• https://treeherder.mozilla.org/jobs?repo=mozilla-release&sea... (click “B” next to your desired platform, OS X in my case)
• https://treeherder.mozilla.org/jobs?repo=mozilla-release&sea... (click “Artifacts and Debugging”, then “target.dmg”/“target.zip”)
• https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task...
… then I get a “Nightly” build reporting version 109.0.1.
The direct links to the latest releases should be better maintained for sure, and it did take me longer than I'd like to figure out the build interface.