When the suite was king, Mozilla decided to create a runtime environment called the GRE (Gecko Runtime Environment). This was going to be a shared platform for building Gecko based applications. The decision was made to stop working on the suite and move to individual applications, Firefox and Thunderbird. The original plan was to have these applications running on top of the GRE. Eventually, that idea was dropped as well and instead the GRE was statically compiled in. Mozilla then decided to create the XULRunner as a platform to build Firefox and Thunderbird on top of. Yesterday, this was scrapped as well.
Why is Mozilla always doomed to repeat its past mistakes
Update: Benjamin Smedberg has a post that helps clarify Mozilla’s stance on the subject.