Although it's only been around for three months, don't get too comfortable with Firefox 4.
Today, Mozilla has updated the Firefox 5 beta to release candidate status (download for Windows | Mac | Linux), which includes improves support for "future-Web" technology, speeds up the browser, and makes multiple smaller tweaks to the browser.
Firefox 5 also disables cross-domain elements as the source for WebGL textures as a response to security concerns involving hardware acceleration. This will break some Web sites and prevent them from resolving properly, although Mozilla says that it is discussing solutions with WebGL developers.
In a minor interface change, Mozilla's Do Not Track header preference has been moved in an effort by the company to make it easier for users to find. Do Not Track is an effort spearheaded by multiple browser makers to get advertisers to respect the desire of people who don't want ads to track them as they browse around the Internet. It works by adding a line to a Web site's meta tags.
Also, the development channel switcher that had been introduced to the About Firefox window has been removed. This was because Mozilla discovered that most of the people using the beta and Aurora channels were running multiple installs of the browser concurrently, Johnathan Nightingale, director of Firefox development, said in a blog post. You can change Firefox development channels at will by downloading and installing each build separately, but you won't be able to change them on the fly from within an already-installed version of Firefox.
The full release notes for Firefox 5 are available here.