Apache on IPfire runs a virtual host on port 444 but also a virtual host serving the proxy.pac on port 81. You have two options, you can create a new virtual server on port 80 serving the same files, or you can use the firewall to redirect the port 80 requests over port 81.
This second option is yet to be documented on the wiki and it is in my to-do list. I do not think this is too difficult to implement, if you would like try this method here is a brief description of how to do it.
EDIT: I entered the missing section in the WIKI. I would appreciate anyone testing that the wpad file is being downloaded when directed to http://ipfire.localdomain/proxy.pac or http://ipfire.localdomain/wpad.dat or http://wpad.localdomain/wpad.dat (where ipfire.localdomain is you local url).