Firefox is based on independent Mozilla Core components layered together. Due to this architecture some of Mozilla’s applications share many components, but they are fundamentally different in functionality.
The Mozilla source code is organized in several branches. The trunk is the main branch, the central source code that is used for continuous and ongoing development. Trunk builds contain the very latest changes and updates. However, the trunk can also be very unstable at times. When development is started for a specific Mozilla version a new branch is created. At conception, a derived branch contains everything that the principal branch contains (Figure 1). Firefox 1.0 branch was derived from Mozilla Branch 1.7 while Firefox 1.5 from Mozilla Branch 1.8. Firefox branches that are forked from the existing branch will be used for all future releases of Firefox. The term release is used in OSS development to refer to different types of releases: major and minor, alpha and beta. Due to data availability constraints, we have only considered two major releases, 1.0 and 1.5, and a minor release, 1.5.0.3, in our work presented here.
Firefox Branch 1.5.0.3 resynchronized the code base with the trunk which contained additional features not available in Firefox 1.0. On the other hand, in release 1.5.0.3 the focus was not on adding features but on improving security related aspects, which were bypassed in version 1.5.0. This characteristic of the three selected releases allowed us to test if the performance of a defect prediction models increases when trained on data collected from major releases instead of minor ones.
The reason behind branching is that components that need to be prepared for a future release are at the same time continuously developed on the trunk. A distinction needs to be made between Firefox-specific source code, i.e. code that does not support any other Mozilla application, and the Mozilla components that support Firefox.
