Funny what you can find on Google. I looked to see if someone had already ported Frontier to Linux. In doing so, I stumbled across the linked page where it apparently was already discussed at some point — making Frontier open source back in 2001, probably under the Apache License. The pages have something on them that says "Confidential" — with a fake red stamp. No idea if that's an internal discussion site of Userland.
There's also a download area with old source code — it still contains an NDA for access to the sources.
Google finds everything.