Despite installing the Australian dictionary, FF is continuiously switching to the US one. A fix that worked for me was to delete the US dictionary.
In Linux you will find dictionaries in /usr/lib/firefox/dictionaries. You will need root privilegies to delete them.
On mac you need to go to your applications folder, right click on FF and select ‘show package contents’, or just go here: /Applications/Firefox.app/Contents/Resources/dictionaries
I found I had to repeat this after FF updates. Can run this to save time:
rm -r /Applications/Firefox.app/Contents/Resources/dictionaries/