Anonymous user names, but some of the more prolific posters might reveal enough information for her to figure out who it is. The firefighter community in Fairfax seems to be tight knit.
Even anonymous users can be traced - every comment has an IP address and even something through a proxy server can be traced. If someone was harassing her, it very likely was on more than one website.
Typically police have to make a request to the host server to get the ball rolling, but it's not impossible to identify "anonymous" online users who threaten others.