Pale Moon is a modified, leaner, meaner version of the Mozilla Firefox browser. It's based on the Firefox 24 code, so it does not have the new interface that many don't like, and features they don't think get much use are cut out, such as webrtc, parental control, etc. It's meaner in the sense that it won't run on old machines without SSE2 support.
You'll have to install any Mozilla extensions you use, as it won't pick up ones that Firefox uses, but it will use the system plugins, such as Flash, gecko-mediaplayer, or pipelight.
This package was rebuilt from the source code in a PPA, so it's not a static binary like our Firefox packages.
Optimized for modern processors
Safe: forked from Firefox and regularly updated.
Supported by a friendly, active community of users
Familiar, efficient, fully customizable interface (no Australis!)
Smooth and speedy page drawing and script processing
Increased stability: experience fewer browser crashes
Support for many Firefox extensions (add-ons) and themes
New: support for Pale Moon exclusive extensions and themes
Extensive and growing support for HTML5 and CSS3
Many customization and configuration options
Able to import existing Firefox profiles with the migration tool