If you make changes to pfSense, the resulting product CANNOT be called pfSense or anything similar. You can call the result any name you like so long as it is distinct from pfSense. As in earlier points, you can state that it's based on pfSense, forked from pfSense, and so on. Stating facts is fine, but creating an association in the product name is not. This is also necessary to protect the trademark.http://doc.pfsense.org/index.php/Can_I_sell_pfSense
"pfSomething", or "somethingSense" -- NOT OK
"ExampleWall", "FireWidget" -- OK
*cough* OPNsense *cough*
IANAL but if I was on a jury I would be inclined to consider you defending "sense" a stronger position than "pf". tcpSense, ipSense, fwSense, etc vs. pfWall, pfEase, pfBarrier, pfPerimiter (?!?) etc. pf existed before the project.