Rated 5 out of 5 stars

https://addons.mozilla.org/en-US/firefox/addon/au-english-dictionary/

https://addons.mozilla.org/en-US/firefox/addon/nz-english-dictionary/

This review is for a previous version of the add-on (2.1.1). 

Rated 5 out of 5 stars

1. Install the following addon: https://addons.mozilla.org/en-US/firefox/addon/au-english-dictionary/
2. Goto: about:config
3. Goto: layout.spellcheckDefault and change the value to 2. Restart Firefox.

This will make the dictionary available for every Firefox profile. For some reason you need to make layout.spellcheckDefault=2 to make Firefox remember the dictionary.

This review is for a previous version of the add-on (2.1.1).