Untangle isn't a fork of pfsense, but it does run on Debian so it would meet OP's requirements. I ran Untangle for quite a while a few years ago, and it's a pretty solid system.
Endian would be another choice. I ran it a long time ago before it went away and came back. Sophos is another good option, heavy on filtering.
These are all leaning more towards the UTM/NGFW, where pfsense can fill that role I don't think it was built that way. I'm a huge fan of FreeBSD though and I installed pfsense once but have never really used it. Go figure.
There is a good reason most router/firewall software is based on BSD. Other than sheer momentum and the fact they they have just forked each others code (.e.g, PFSense/OpenSense, etc.).
Most of these platforms rely on OpenBSD Packet Filters, which until recently only existed in the BSD kernel. PF is the "PF" of "PFSense". OpenBSD PF provides a very efficient, very easy to use, method of implementing high(ish) performance networking and firewall applications.
Recent capabilities in Linux (Berkely Packet Filters added to the Linux kernel) as well as user-space solutions like VPP have emerged that largely negate BSDs advantages for firewall applications and - given the larger usage base of Linux we are likely to see some Linux based router/firewall applications starting to mature and gain the momentum in the marketplace needed to overtake BSD based solutions.