Firefox scans other locations for dictionary files on Linux.

You can check these locations for globally installed dictionaries:

/usr/share/hunspell/
/usr/share/myspell/

I see some problematic info ("und" = undefined) reported in the troubleshooting information. So Firefox could have a problem with the default locale reported by the OS.

osPrefs:
regionalPrefsLocales:["en-AU"];
systemLocales:["en-AU"]; 
localeService: available:["en-GB","en-US"];
requested:["en-AU","en-US"];
regionalPrefs:["en-AU"];
supported:["en-US","en-GB","und"];
defaultLocale:"und";

See also these prefs:

• intl.locale.requested = "en-AU,en-US"
  (you can try to create this String pref)
• intl.regional_prefs.use_os_locales

Malvineous said

In my Firefox installation folder (/usr/lib/firefox) I can only find files for en-US and the en-GB language pack. In my own profile folder I have the en-AU add-on, but that is all. about:addons only shows the en-GB pack and en-AU dictionary

So you know, the Language Packs are used to change the language of the user interface and has nothing to do with the inline spellchecker. https://support.mozilla.org/en-US/kb/use-firefox-another-language

The screenshot shows you have a en-GB language pack (3) and the en-AU dictionary (2).

What is the default Firefox locale you have installed? Is that the en-US Firefox version that includes the en-US dictionary?

If you do not need the en-US dictionary then you can possibly install the en-GB version that doesn't come with a dictionary.

That was the problem! My Linux distro (Arch Linux) for some reason installs the en_PH dictionary when en_US is installed, in <tt>/usr/share/hunspell</tt>. Firefox was picking this up and somehow choosing it as the default.

I wasn't able to change the default - even setting those <tt>about:config</tt> settings to en_AU, however with en_PH now removed, it seems to default to en_AU now which is perfect.

Many thanks for your help!