Mozilla's forthcoming Firefox 3.1 browser will include a Private Browsing Mode that prevents Web site visits from being recorded by the browser.

This feature, informally known among many Internet users as "porn mode," has been on the Firefox development road map for more than four years, since Apple's plan to include private browsing in its next release of the Safari Web browser became known in 2004.

But the launch of Google's Chrome browser last week appears to have prompted Firefox's developers to be less leisurely in bringing the private browsing feature to market, lest Firefox look like a laggard. Like Safari and Chrome, Microsoft Internet Explorer 8 beta 2 includes a private browsing mode.

Three days after Google announced that it was getting into the browser business with Chrome, Alex Faaborg, user experience specialist at Mozilla, in a post on Mozilla's bug tracking site said, "Recent development with Chrome will likely make finally getting private browsing mode shipped a priority for 3.1, but I think we are now targeting a more lightweight implementation."

Private browsing typically involves discarding or blocking browser cookies, Web history data, Web searches and stored form data. It provides a way to shield one's Web activities from others on the same computer; it does nothing to prevent records of the user's visit from being recorded remotely.

Mike Beltzner, director of Firefox development, said the Firefox team is looking to address some of the shortcomings of other private browsing implementations. He explained that most browsers load cookies into memory to make Web pages load more quickly and that entering into privacy mode with cookies in memory leaves an opening for those cookies to still be tracked.

Also, he said that while other privacy modes are modal, either on or off, they're not as good at dealing with a situation where a user wants to apply privacy mode after the fact.

Beltzner was quick to point out that privacy mode is one of Firefox's older features and that "Mozilla has always been really interested in giving users full control over their private browsing mode." He also noted that Firefox was the first browser to have a button for wiping private data.

But implementing privacy mode proved difficult, due to some architectural issues with Gecko, the rendering engine used by Firefox, Beltzner explained. Safari and Chrome use a rendering engine called WebKit.

With Firefox 3, the Gecko issues became less significant and private browsing is back on the front burner.

Beltzner acknowledged that developments in other browsers get discussed. "We do keep a close eye on what other browsers are doing," he said.

But he also pointed out that due to the open nature of Firefox's development, competitors can keep a close eye on what Firefox contributors are doing.

Beltzner warned about the risks of feature-based competition between browser makers. The last time that happened, he said, browsers got lots of features that weren't useful.

Firefox 3.1 should appear in beta at the end of the month.