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Not sure why you’re getting this warning, but the package has been removed a while back and is no longer associated with Firedragon.
There is no xdman required, nor firedragon-extension-xdm-browser-monitor.
https://aur.archlinux.org/packages/firedragon?all_deps=1#pkgdeps
So you can remove both xdman and firedragon-extension-xdm-browser-monitor which should clear out the deps.