When it installs then your hacking of the extension worked as intended since you only changed its version refusal.
Still it is possible the extension itself - even when it is able to be installed into your browser - will not work or not work as intended. The hacking of install.rdf and re-compressing the xpi is an easy fix, but it is not a 100% successful fix.
Might be the original extension creator excludes your browser version because he (or she) knows it will not work as intended, or it is solely for Firefox and known to not work with Palemoon, or dozens of other reasons the creator choose to not let the extension be able to be installed on your browser or on the specific version of your browser.
In my book you have 2 choices left:
● find another extension as alternative or
● find a specialized forum about that very extension where users who also tried to get it running by hacking its install.rdf can tell you how they managed it in the end. Or a Firefox forum that discusses lots of extensions hacking, or a forum for Firefox forks… it doesn't need to be this Palemoon forum. Or a forum for hacking all kinds of browser extensions… be they Chrom[ei]* or Firefox or whatever-browser-based.
Or you manage to convince someone on here to do extended hacking (more than editing the versions requirement via install.rdf) and attempt to get it working on a PM version near or identical of your own.
Similar like existing open and free software extensions have been tweaked from their Firefox variant into one that works for Palemoon.
Sorry fatboy but that's all I can think of.