Firefox 5 muda em palavras simples
Mesmo que a GUI não tenha sido alterada, o Firefox 5 apresenta:
Better standards support for HTML 5, CSS3, MathML, XHR and SMIL
Better visibility for the Do-Not-Track header preference
Stability and security improvements
WebGL security has been tightened up.
Better tuned HTTP idle connection logic
Improved canvas, JavaScript, memory, and networking performance
Improved spell checking for some locales
Improved desktop environment integration for Linux users
WebGL content can no longer load cross-domain textures
Background tabs have setTimeout and setInterval clamped to 1000ms to improve performance
Via Wikipedia