esruser said

The following URL is supposed to work: https://download.mozilla.org/?product=firefox-esr-latest&os=win64&lang=en-US However, I am presented with a 404.

Not a valid url since there are no Win64 Firefox 38.0.x Releases let alone Firefox 38 ESR Releases. The first Firefox Release to have Win64 Firefox was 42.0.

There is no Win64 versions of the older Firefox 38 ESR branch. However there are Win64 builds of Firefox 45.0 ESR (like with current Fx 45.0 Release).

Both current Firefox 45 ESR and 38 ESR versions are listed (look at top for how Firefox 45 Firefox 38) on https://www.mozilla.org/firefox/organizations/all/ however the Win64 Firefox 45.0ESR is currently not listed on page.

https://ftp.mozilla.org/pub/firefox/releases/45.0esr/

There is a two-version transition period for users of ESR-38 to test ESR-45 and ready their systems for it. I think that explains why the download server does not push 45 right away, on the assumption that those using it will want that time.

If you prefer 45, you can download from this page: https://www.mozilla.org/firefox/organizations/all/ (above the list on the right is a selector for 38 or 45 links).

Regarding the download link, I don't know why it isn't working as described. There is a mailing list for "release engineering" that might be a place to mention this problem: https://lists.mozilla.org/listinfo/release-engineering.

I will contact the release-engineering list with other inquiries and about this one. Thank you both for your replies.

Happy Surfing!