Add hunspell settings for firefox.

Taken from the latest ubuntu.
 https://bazaar.launchpad.net/~mozillateam/firefox/firefox.bionic/view/head:/debian/vendor-gre.js

Author: tsutsui

liveimagefiles/firefox/defaults/pref/local-prefs.js

@@ -3,3 +3,6 @@ pref("intl.locale.requested", "");
 
 // Enalbe Network Manager integration
 pref("network.manage-offline-status", true);
+
+// load system dictionaries
+pref("spellchecker.dictionary_path", "/usr/pkg/share/hunspell");