From http://www.mozilla.com/firefox/releases/1.5.0.1.html:

February 1, 2006. Firefox 1.5.0.1 is a stability and security update that is part of our ongoing program to provide a safe Internet experience for our customers. We recommend that all users upgrade to this latest version.

Here’s what’s new in Firefox 1.5.0.1:

• Improved stability.
• Improved support for Mac OS X.
• International Domain Name support for Iceland (.is) is now enabled.
• Fixes for several memory leaks.
• Several security enhancements.

This sounds like a must-have upgrade, especially if you’re from Iceland! 🙂