Respuestas:
Las notas de la versión están disponibles aquí (demasiadas para enumerarlas aquí):
Y aquí está la lista completa de errores corregidos.
Firefox 5 cambia en palabras simples
Aunque la GUI no ha cambiado, Firefox 5 presenta:
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
Vía Wikipedia