The engineers at Mozilla have added a new feature to popular open-source Web browser Firefox that many users felt was missing. Following in the steps of Microsoft’s Internet Explorer, Apple’s Safari and Google’s Chrome browsers, Firefox now has an enhanced privacy browsing mode affectionately known as “porn mode.”

Released Monday night in the latest build of Minefield, adding porn mode to Firefox has long been in the queue for Firefox, even though the feature was not included in Firefox 3’s initial release in June.

Porn mode, simply put, allows users to turn on the privacy feature in Firefox and not have the browser capture the latest URLs or cookies that you’ve visited on the Web. This means that some users can surf the Web for questionable content and not worry about anyone else sitting down at the machine and sussing out what you’ve been looking at.

This isn’t the first time that Firefox users have been able to surf the Web without leaving cookies or a history in the browser. In fact, Stealthier has long been available as a plug-in for Firefox. The difference, however, is that porn mode is now natively built into the browser, which means a plug-in is no longer required.

It’s likely that a privacy browsing mode will be built into Firefox 3.1, along with enhanced privacy features and performance features.

Mozilla has been having a good week all around. Earlier in the week the makers of Firefox announced that they had crossed 20 percent of Web browser market share for the first time, according to NetApplications. The uptick in Firefox’s popularity comes at the expense of Microsoft’s Web browsing share, which has declined.

The Firefox crew is rightly excited about crossing the 20 percent threshold because it shows that a small, open-source company can play on the same field as the big corporations. And right now, Mozilla is taking it to Microsoft on its own turf—and though they’ve added a new layer of privacy, Firefox is doing it right out in the open.