It probably has to be NPAPI plugin, since it is for Pale Moon users specifically after all, and I'm not sure if extensions have the capability... I wonder why they would opt for their own client though (whether it's a webpage, plugin, or installed application) instead of using a standard protocol like IKEv2 or L2TP/IPsec which should not require any VPN client at all.

We probably should redirect the https://linux.palemoon.org/download/mainline/ page to the current download page at https://www.palemoon.org/download.shtml though. The linux subdomain is outdated...