Don't know about Powerfox / Basilisk, but Palemoon developers have outlined How are security vulnerabilities handled? - https://forum.palemoon.org/viewtopic.php?f=24&t=23577 - in their FAQ. As both Palemoon and Basilisk do share codes from each other, as they started from a common codebase, I am guessing that most Palemoon fixes also apply to it.

(Then again, Powerfox is meant to run on OSes that have been EOLed for more than a decade now ... so I am not sure how secure a browser, any browser, will be running on it. :)