There's a whole load of people who're still using Firefox 2, despite the vastly superior version 3 having been released more than a year ago. Mozilla's security team wanted to know why, so they did a survey of people who consistently declined updates. The reason?

Porn.

The number one excuse that people gave for not upgrading was because they were worried about Firefox 3's "Awesomebar", which replaces the address bar and offers you the option to search against history and bookmarks.

People didn't want iffy websites popping up when they were browsing in front of other people, says Mozilla. "In some cases users had intentionally hidden these bookmarks in deep hierarchies of folders, somewhat similar to how one might hide a physical object", said Firefox designer Alex Faaborg.

"Having something from your previous browsing displayed to someone else who is using your computer (or even worse) to a large audience of people as you are giving a presentation, is really one of the most embarrassing things that Firefox can do to you".

In the most recent iteration of the browser, Firefox 3.5, Mozilla introduced a "private browsing" mode, dubbed "porn mode" by many, which masks your browsing and doesn't save any history or cache.