Based on my engagement over the past (closing in on 4 years) I have previously seen request for Waterfox.
Manjaro uses - mostly - the packages from the Archlinux repo - and only a few packages are picked from AUR - and then only if a maintainer deems the package necessary for some desired functionality on any given edition.
As you cleverly point out Waterfox do not have the necessary userbase and none of the team members has a knack for it - so I don’t think it is going to happen.
But - as you say - one wouldn’t know if one don’t ask.
But you could take a look at the archive - search phrase waterfox
If you issue is privacy then Palemoon is in the community repo.
sudo pacman -Syu palemoon-bin