According to various reports, the “private mode” feature that was first introduced in Safari is now going to get implemented in Firefox as well. It was originally planned for version 3.0, but was delayed to a later release; now that all major competitors such as Chrome and IE 8 are going to get this, it’s back on the list for inclusion in version 3.1.
Most people call this a “porn button”, but it can actually be useful on other occasions. You can use it on public computers where you don’t want other people to go through the websites you visited, or for testing a site that sets cookies. Enable private mode, test your stuff, and when you start a normal session again all your cookies are still there, except for the ones set by the site you were testing. I like the Chrome implementation better than the one in Safari, because you can enable it for a single tab instead of the entire browser, but I’m sure this feature will be in a Webkit build in the near future.
For those of you that have an urgent need for this feature: try the Stealther plugin, it basically does the same thing the “private mode” is supposed to achieve.