What's new:

Browser:
# Official Mozilla 1.7 Beta installer builds (including the OS X disk image) contain the Talkback crash reporting utility. To help us make 1.7 the most stable release yet, please submit your crash reports.
# A new option to prevent sites using Javascript to block the browser's context menu.
# A new set of icons for files that are associated with Mozilla.
# Password Manager has a "show passwords" mode which will display saved passwords. You will need to enter your master password if you are using one.
# The "Set as wallpaper" feature now has a confirmation dialog.
# Linux GTK2 builds have improved support for OS themes.
# Cookie dialogs have been reworked to make them more usable.
# Date handling, especially on OS X, has been improved.

Mail:
# Many improvements to Palm Sync.
# IMAP IDLE support has been added.
# Support for "MSN Authentication" and Secure Password Authentication using SSPI NTLM auth for SMTP and POP3.
# A new preference to "always use the default character encoding for replies" rather than using the encoding of the message being replied to.
# Improvements to performance of downloading, viewing, and saving mail messages.

Chatzilla:
# Chatzilla now supports zooming of fonts with keyboard shortcuts (Ctrl + and Ctrl -) as well as with the View menu.
# Improved date handling; using the date/time format for the locale.

Under the Hood:
# Mozilla 1.7 size and performance have improved dramatically with this release. When compared to Mozilla 1.6, Mozilla 1.7 Beta is 7% faster at startup, is 8% faster at window open time, has 9% faster pageloading times, and is 5% smaller in binary size.
# A long-standing bug with CSS backgrounds on table elements has been fixed (standards mode only).
# Support for Kerberos HTTP authentication using GSSAPI (benefits Unix-like platforms including Linux and OS X).
# Support for smb:// URLs using the gnome-vfs library (only enabled in GTK2+XFT Linux builds).
# Support for server push of XML documents using multipart/x-mixed-replace and XMLHttpRequest.
# Liveconnect now works when a Java applet's codebase is in a different domain.
# Very wide images (more than 4095px) will now display on Mac.
# Support for the CSS3 opacity property.