The main sentence is
Secundo firmamentum caeli in medio libravit aquarum 'On the second day he poised the firmament of the Sky in the middle of the waters
Matthew Paris now begins to back-track by inserting an enormous ablative absolute:
ipsis aquis creatis 'those same waters having been created (past)'
He then elaborates by describing what else was created at the same time:
ipsis aquis ac terra cum caelo superiore ac virtutibus -all of them Ablative, all jointly Subject of the Ablative Absolute.
And when they were created:"Before the...
ante horum sex dierum exordium
And what the superior heavens and the virtues (and the waters) were doing before the Six Days:
quae in ea conditorem laudarent, ante horum sex dierum exordium
Thanks to cnread re: libro