The most crufty part in both mozilla's code base and libreoffice code base is essentially equivalent in that it tries to solve essentially same set of problems

The most crufty part in both mozilla's code base and libreoffice code base is essentially equivalent in that it tries to solve essentially same set of problems (cross-platform UI and object model), but the implementation is significantly different, because in mozilla's case this layer of cruft was introduced 20-ish years ago as a complete rewrite of the thing (and most of the really ugly hacks have to do with sub-par C++ compilers and support for classic MacOS), while there was no attempt at a major rewrite of the staroffice codebase until libreoffice (and it is quite obvious that the way libreoffice's rewrite proceeds is at least partially motivated by experience from how the netscape->mozilla rewrite suffered heavily from second system effect and in the end failed at least from the bussines standpoint).