Pale Moon is an open-source web browser forked off Firefox/Mozilla code, now fully independent.
Ranked in these QuestionsQuestion Ranking
Pro Has its own add-on ecosystem, built on time-tested technologies such as XUL (plus JS and CSS) and XPCOM
"Classic" Firefox add-ons can work, but they are not supported and should be updated or forked to become a Pale Moon add-on.
Pro Designed for usability, not the shiny new things
Pro Light on resources, although it's not its main focus
Pro Respects your privacy
Is not spyware like Chrome or Firefox.
Pro Independent from Google and Mozilla
Pale Moon is based on some old Firefox code, but it's a completely different browser.
Pro Open Source
Pro Support for existing web standards
Pro Has its own library of legacy extensions
Pro Supports complete themes
Pale Moon supports complete themes, something which Firefox used to have before version 57.
Pro Good community support
Pro Uses Goanna layout engine
Unlike most other browsers, Pale Moon uses its custom engine.
Pro Optimized for modern processors
Pro Legacy Firefox
Pale Moon lacks the sandboxing and other privacy protecting features included in latest Firefox releases.