Why xfce4-notifyd?

[Tom Horsley]

When firefox pops up a notification (like all downloads are
complete) it shows up in my fvwm session as a window
titled "xfce4-notifyd".

Where the heck is the xfce stuff coming from? I'm not
using xfce, I'm starting fvwm with my own ~/.xsession
script which doesn't make a single mention of anything
xfce related.

Why is firefox unable to popup its own notification
windows?