The problem with Pale Moon is it's effectively a dead end from a development standpoint. Yes, it gets security fixes from upstream (for which are applicable) but it doesn't receive much in the way of meaningful web compatibility (the elephant in the room being WebComponents, which none of the team have the capability of implementing in the current codebase). It's based on Firefox 52 and refuses to re-base on later versions, partially due to lack of XUL-based extensions and partly due to the introduction of Rust (which does not exist on some platforms which Pale Moon supports - including Solaris/Illumos).