main developer of pale moon (moonchild) is hostile and took down mypal, a fork of pale moon
based on ancient codebase of firefox
doesn’t fully support CSS3 compared to current versions of Firefox and Chrome, sites may not look correct
security risks
javascript makes pale moon slow down
if the only reason for using pale moon is for the classic theming engine(?), then I’d recommend you ethier fiddle with Firefox‘s userchrome.css or you wait until Seamonkey finally gets an updated version of Gecko (discord looks broken on latest version of seamonkey due to over-engineered css math and the issue got fixed in an ALPHA BUILD of the next version)