Nice guys use good old Internet Explorer, geeks use Opera and NSFW addicts use Firefox.
According to DownloadSquad, there is some truth in that joke:

In fact, I'd go as far to suggest that porn surfers -- or at least the search for naughty, illicit, adult-only material -- account for Firefox's biggest market demographic. The 'young' Internet -- blogs, social networks, porn -- is predominantly ruled by the spotty teenagers of the browser world: Firefox and Chrome. Internet Explorer's total market share is larger, but corporations, geriatrics and aging family computers -- i.e. the non-porn world -- account for most of that.

Finding adult-only material is not quite easy for everyone, but if you are using Firefox as your default browser that means you have at least some basic knowledge about the internet.