Breaking Guinness World Records ain't that hard, really: you just need to invent a new one. For example, "most software downloads in 24 hours," as that's the record Firefox wants to break with the launch of Firefox 3.
The actual date of the launch is, unfortunately, yet to be announced, but when it comes, you better be ready for some heavy duty downloading. Breaking this particular world record is not a small feat; I can remember several pieces of software which got downloaded a lot in a short timespan, for example World of Warcraft's latest expansion.
Although you cannot actually download anything yet, you can pledge that you're ready to help them break the record. Over 40.000 people have already pledged, but I can imagine this number will grow significantly in the following days. You can even see how many people have pledged from each individual country on a nifty world map, as pictured below. So far, the US is in the lead, with Spain and China holding the second and third place.