Internal project names

Most major versions of Mozilla software have codenames. Each time it is decided to create the next major version of software, it is split from the main development tree (in a process called "branching") and is given a codename. These "nicknames" are given to differentiate the product from other versions before being officially named, numbered, and released. Typically, any subsequent minor releases (that include only security fixes and small improvements) use the same codename and branch as before.

Codenames

Phoenix 

Firefox 1.0. Also, the official name of very early (0.1 - 0.5) versions of what became Firefox.

Firebird 

the offical name of what became Firefox from 0.6 to 0.7.1.

Deer Park 

Firefox 1.5, which was initially scheduled to be Firefox 1.1.

Bon Echo 

Firefox 2 development, which is to be the next major version of Firefox.

Minefield 

the trunk (main development branch). Work done here currently will be released with Firefox 3.

Lightning 

a project working to integrate Thunderbird with calendar functionality.

SeaMonkey 

former codename of the Mozilla Suite but has been adopted as the official name for its successor.

Shredder 

Thunderbird 3, and also Thunderbird trunk

Gran Paradiso 

Firefox 3

Suffixes

Final 

pseudonym for the production release of a product. Although not actually part of the name, it indicates that the product has been sufficiently tested and is acceptable for use by the public and companies. Any future fixes (possibly required due to security bugs) would result in a minor version number increase using the same product name.

Release Candidate 

may become the final version (or official test version) of that product, if no unexpected major bugs are found.

Preview Release 

to be used for general testing and is mostly feature complete - containing all of the new features intended for the product.

Beta 

a fairly stable test version of the product.

Alpha 

an early test version of the product. May contain obvious bugs and incomplete features.

Nightly 

a developer version. May contain serious bugs and incomplete features. Recommended for testers and developers only. It's recommended that before you file a bug, you see if the bug occurs in the latest nightlies.

Other Codenames

Earlier development versions of Firefox had other codenames, which you can see here