LibreOffice is a fork of the Open Office software, one meant to bring in better features and stability. It shares much of the same code, but the developers have retooled plenty.
The first byproduct of this new project will be a document viewer running LibreOffice's core. That document viewer will allow for plenty of testing of the rendering engine before the eventual full suite comes out. Hopefully it will work out most of the bugs before editing is even worked on.
Interestingly, the LibreOffice Android project is going to be using some of Mozilla's code. Mozilla managed a fairly interesting feat, being able to link to an external APK and load libraries from it. The idea is that LibreOffice can have an APK to pull data from only as it needs, loading editing capability, the routines for slide editing, etc, only when needed. This isn't something that apps normally are able to do, so even if Mozilla's browser doesn't live on, this technique probably will.