Mozilla, which launched the latest beta of Firefox 4 last week, has started to drop features from the still-under-construction browser.

Firefox 4 Beta 5 shipped Sept. 7, and included support for a new audio API (application programming interface) that allows developers to tap raw audio data from within the browser, as well as support for HTTP Strict Transport Security, a Web security protocol that lets site designers force Firefox to automatically use a secure connection.

The latter is meant to help stymie "man-in-the-middle" attacks, in which hackers essentially eavesdrop on users' Web traffic -- most often at public Wi-Fi hotspots -- in the hope of snatching clear-language transmission of passwords or credit card numbers.

Firefox 4 also switched on Windows hardware acceleration by default in Beta 5; Mozilla had included the technology in August's Beta 4, but had left it turned off, requiring users to edit the browser's "about:config" file if they wanted to try it out.

Firefox 4, like rival Microsoft's Internet Explorer 9 (IE9), relies on Windows' Direct2D API to boost browser performance by shifting some chores from a computer's central processor to the graphics processor.

The hardware acceleration switched on in Beta 5 requires Windows Vista or Windows 7; the more popular Windows XP lacks the necessary graphics infrastructure, a fact that's prompted Microsoft to drop XP from IE9's supported operating systems.

Mozilla isn't going to that extreme. Although Firefox 4 won't boost content rendering in Windows XP, Mozilla does plan to increase the speed of "compositing" -- the process of assembling the various pieces of a site -- in the nine-year-old operating system by leveraging the Direct3D API in a future preview.

But as Mozilla's self-imposed deadline for building a feature-complete beta nears, the company has also started dumping features it once hoped to squeeze into the upgrade.

First to go was Account Manager, which Mozilla ditched late last month.

The beefed-up password manager was supposed to take full responsibility for site sign-ons, relieving users of the chore of remembering and then entering various usernames and passwords. An experimental add-on is still available, but only works with a limited number of sites, including Google, Facebook and Yahoo!