Mozilla has a code name for each of the major Firefox builds usually named after national parks. Firefox 3.6 was Namoroka (National Park in Madagascar) while Firefox 4.0 is Tumucumaque (National Park in Brazil). Starting with Firefox 4, each release is going to have its own animal code name. Keeping with the Amazon theme, the next main update for Firefox 4 is going to be Macaw. You could say this would be Firefox 4.0.1, but Mozilla rather you not. They are trying to get away from the numbering as any time there is a security update (aka Firedrill release) this messes up the numbering and causes major headaches.
Suppose a major exploit in Firefox 4 is discovered over the weekend. Mozilla gets a patch out the following Monday. Under the traditional naming system, this would become Firefox 4.0.1 and everything else that was targeted for 4.0.1 would be moved to 4.0.2. This causes a lot of issues with developers and makes a mess over at Bugzilla. Under the new naming system, the patch would be called Anteater (yes, that is the planned name if there would need to be a security patch). The next planned update would still be Macaw and would still be released as scheduled (currently April 26th).
Under the new naming system security updates will be named after an animal being with the letters A-L. Subsequently planned releases will be named after an animal beginning with the letters M-Z.
This new naming scheme is going to take some getting use to, but it really does make sense. You can learn more here.