This is purely a trademark dispute. They want you to rename the browser if you deviate from their franken-patchset of firefox + all the other weird old libraries they use. Such a thing would obviously not fly in a security focussed OS like openbsd (or any OS for that matter). So you can rename the browser if an agreement is not reached, which is fine. The issue, however, is that these guys are so protective of their name (really, who even has heard of it?), that they thwarted a WIP attempt by someone to port this mess before the ported product even saw the light of the day.
The astonishing thing to me is that, as mentioned elsewhere, this mess in the GitHub issue happened after the person doing the port politely asked the Pale Moon people about the right way to do things with regards to patches and branding: https://forum.palemoon.org/viewtopic.php?t=18256
