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 Independent from Google and Mozilla
Pale Moon is an independent fork of an older version of Firefox. Therefore, it is independent from Mozilla and are not affected by their terrible decisions such as removing XUL, adding telemetry, pocket, etc.
Pro Respects your privacy
Contains much less spyware than Chrome and Firefox and all of it can easily be disabled.
Pro Support for existing web standards
Pro Open source
So we can verify that the browser is not spyware.
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 Support for GTK themes
Pale Moon supports your GTK theme while Firefox does not.
Pro Uses Goanna layout engine
Unlike most other browsers, Pale Moon uses its custom engine.
Pro Optimized for modern processors
Pro Good community support
Pro Legacy Firefox
Pale Moon lacks the sandboxing and other privacy protecting features included in latest Firefox releases.
Con Uses Goanna
It an old Gecko-fork that is developed mainly by one man.
Con Outdated rendering engine
It is an really old fork of Gecko that misses many of the newer web features.
Con Pale Moon is based on very outdated Firefox code
Con Lacks popular extensions and adblockers
Though it used to support Firefox extensions, it no longer does. The best supported ad-blocking extension, uBlock Origin, is mediocre at best, falling short of AdBlock Plus.
Con Still contains some spyware
default homepage is spyware and search suggestions and automatic updates are enabled by default.