So whats new?
Good question, overall you won't see much in terms of UI changes. Its really whats under the hood that matters most. Here are the release notes that will give users all the info they might want or need.
Firefox 5.0 has the following changes:
- Added support for CSS animations
- The Do-Not-Track header preference has been moved to increase discoverability
- Tuned HTTP idle connection logic for increased performance
- Improved standards support for HTML5, XHR, MathML, SMIL, and canvas
- 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
- Fixed several stability issues
- Fixed several security issues
You should not that even though this is a minor update some of your add-ons might not support the new version. Its not that they won't work but the authors need to update support for the new release. So you might want to hold off for a day or two, or check with the add-ons site to see yours has been updated.