To change your setting in Firefox...
Click on the "App menu" (The 3 horizontal lines at the top right of your browser)
Go to "Settings" and scroll down to “Languages” and select “Choose”.
Click on “Select a language to add...” and scroll down until you see "English (New Zealand) or English (Australia)" and click “Add”.
Make sure your language selection is sitting at the top of the list or click "Move up" until it’s at the top.
Click “OK” and then you can close down the settings page.