So, I found how to fix it. The problem isn't in the dictionary; it's in Firefox refusing to un-Americanise itself. Solution:
Download Agent Ransack. You need this to search for files properly. Search the term "dictionaries:. You'll find a few folders with that name. One of them will contain only US English files ( en-US.aff and en-US.dic ) if your Firefox refused to use the Australian dictionary. For me the folder was C:\Program Files (x86)\Comodo\IceDragon because that's the version of Firefox I was trying to fix. Open the folder.
Also open the folder that DOES contain your Australian dictionary files ( en-AU.aff and en-AU.dic ). For me that was in an extensions folder, among other places.
Now, copy your Australian dictionary files into the folder containing your American files. Almost finished.
Next, to be sure, rename your American files. I added .was to give them a new file extension that would be easy to reverse if necessary.
NOTE: You'll need administrator access to move the files. You'll need to have file extensions visible (set that in Folder Options control panel).
This review is for a previous version of the add-on (2.2).
This user has a previous review of this add-on.