Firefox is in the news again this week, with two major issues in the spotlight. First of all, the browser has reached a new milestone, by passing the 20 percent threshold market share for the first time last month, as reported by Net Applications.

Mozilla Firefox still has a long way to go before reaching the market share of rival Internet Explorer, but we can’t help but noticing the remarkable progress that browser has made in almost four years of existence.

With its popularity still growing, Mozilla announced this week that a new feature will be available for testing, one that has been much discussed before Firefox 3.0 came out: the Private Browsing (a.k.a. “porn mode”).

As developer Ehsan Akhgari noted on his blog, the Private Browsing feature basically aims at helping users “leave no trace” of their browsing activities on their computers. However, he explained, this is not a tool for anonymization from websites and ISPs.

“Private Browsing is only about making sure that Firefox doesn’t store any data which can be used to trace your online activities, no more, no less,” Akhgari wrote. This means that whenever you close your browsing windows, the Private Browsing feature will ensure nobody else but you will know what you did during your browsing session.

The feature, which was supposed to come out at the same time with the summer launch of Firefox 3, had to be delayed, but we should expect to find it in Firefox 3.1, after the testing period is over.