New End User Features:
"Sanitize" privacy feature
The "Sanitize" feature provides an easy way to quickly remove browsing history, cookies, cache, saved form information, and other personal data. The items to be removed can be customized, and the feature can be activated using either a keyboard shortcut or through a menu item.
Image thumbnails as tab icons
When viewing images, tab icons now display thumbnails of the displayed image.
Fast back (and forward)
This very experimental feature allows much faster session history navigation. The feature is off by default but can be enabled for testing purposes by setting the browser.sessionhistory.max_viewers preference to a nonzero number.
Anonymous FTP login failure behavior
FTP users are now prompted to input a name and password if anonymous access fails.
CSS at-rule for matching on site/document URL
The new @-moz-document rule gives users the ability to match page objects per-site, using CSS. This makes it possible to include site-specific rules in user style sheets (userContent.css). David Baron's post to www-style explains how the rule can be used.
Report a broken website wizard
Users now have a simple tool for reporting websites that aren't working in Firefox. If installed (Reporter is a custom install option for this release) the tool can be accessed from "Report Broken Website" on the Firefox Help menu.
New Web Developer Features:
Elements with tabindex="-1" should be focusable
Elements with a negative tabindex attribute can now have focus, even though they are not in the tab order.
Object should submit
In accordance with the HTML4 specification, <object> elements can now be submitted as part of a form.
CSS2 quotes nesting
Starting with this release, the CSS2 quotes property is fully supported, with the correct quote (depending on the nesting level) being used for open-quote and close-quote.
This CSS3 selector allows selecting an element that has no other elements as siblings in the DOM.
An experimental implementation of the proposed CSS3 multicolumn layout draft. This allows easily doing newspaper-like multicolumn presentation.
CSS3 overflow-x and overflow-y properties
These properties can be used to control the overflow behavior in the horizontal and vertical directions somewhat independently. For example, overflow in the horizontal direction could be hidden while overflow in the vertical direction can be scrolled to.
More mouse cursor names are now supported.
URI values on CSS cursor properties
On Windows, OS/2 and Linux (Gtk+ 2.x) one can now use an arbitrary image as the mouse cursor while a given DOM node is being hovered. Any image format supported by Gecko can be used for the image (SVG, animated GIF, and ANI cursors are not supported).
CSS outline property
CSS outlines can now be used. These differ from borders in that they don't affect the page layout.
CSS outlines can now have rounded corners.
Counters in CSS-generated content
CSS2 counters are now completely supported (the implementation doesn't match the current CSS2.1 draft, but matches the upcoming one). This allows automatic numbering of sections, headings, and so forth via stylesheets.
Text written in new documents created with document.open("text/plain") is now treated as text rather than HTML, so line breaks will remain intact and tags will not be parsed.
"XML Events" is a W3C specification to provide XML languages with the ability to integrate declarative event listeners and event handlers.
Canceling the keydown event now properly cancels any corresponding keyup/keypress events, per the DOM specification.
Accessibility APIs for DHTML
Mozilla now allows DHTML authors to add role and state semantics to custom elements, and exposes that information via MSAA and ATK.
DHTML Performance Fixes
A number of changes were made to significantly improve DHTML speed and smoothness.
SVG is W3C specification providing resolution-independent scalable vector graphics, along with a DOM. A technology preview of native SVG support is included in this release. Currently a subset of SVG 1.1 Full, missing functionality includes filters, declarative animation, and SVG defined fonts.
<canvas> is a scriptable bitmap drawing surface. It is suitable for many uses, such as adding graphs to interactive DHTML and drawing game graphics. For a further introduction, see Drawing Graphics with Canvas.
Support HTTP/1.1 408 response code
A persistent connection is now correctly closed when a 408 response code (Request timeout) is received. The request is retried in a new connection.
URIs always sent as UTF8
URIs are now always sent to the server as UTF8, regardless of the linking page's encoding. This fixes images and links on sites with non-ASCII filenames.
The W3C's XML Forms language allows writing complex forms in XML, and includes features that regular HTML forms do not have, such as client side validation against XML Schema and XML submission/retrieval. Support for XForms comes as an extension, see Mozilla XForms Project Page.