This is where you find if it's supported.

https://www.mozilla.org/en-US/firefox/new/?experiment=download-as-default&variation=treatment

That delivered an .exe installer, I'm looking for an MSI installer I can push out via inTune.

I can wrap the .exe into an intunewin file using the Microsoft Win32 Content Prep Tool - but if there is an arm64 MSI installer available it saves a lot of extra work on my end.

90% of the other software I use has MSI installers for both arm64 and x64

That's where one goes to get Firefox browser installer.

This is where one goes to get the various flavors of ESR installers

https://www.mozilla.org/en-US/firefox/all/desktop-esr/

My experience with other vendors has shown that just because they don't have an MSI installer on their public facing download page, does not necessarily mean it does not exist.

For example, the unofficial nightly build contains an MSIx file I can use - answered my own question :)

https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/firefox-138.0a1.multi.win64-aarch64.installer.msix

The Nightly development channel can get checkins and updates twice a day in testing though.

If there is another source for a stable arm64 MSI or MSIX installer (ideally in the ESR branch), I'm all ears.