Did you zipped all files and folders into one zip file
(content,defaults,locale,skin,chrome.manifest,install.rdf) ?
Avast online security in FF folder is only unpacked addon.
By zipping the content of FF folder and changing zip extension to xpi,
you're just packing it as addon.
What version of Pale Moon are you using ?
I tried one more time with Pale moon 20.1 and everything is OK.