Comment 45 for bug 476853

Couldn't get Firefox (31.0) to work, but used this with Rekonq as a workaround:

Add this line:
application/x-deb=AptURL.desktop;

to:
~/.local/share/applications/mimeapps.list

Works with Kubuntu 14.04 LTS and using Rekonq to access PlayDeb for me.

Thanks to:
http://askubuntu.com/questions/279161/how-to-open-deb-from-nautilus-with-apturl