Excellent that we have this dictionary. Shame we have to add it as an add-on rather than coming as standard in Firefox! I realise that Mozilla is a US organisation but think it's reasonable to expect fuller language support. There are some errors in this dictionary though: I noticed that words ending -ize are treated as correct whereas that is invariably the USA form, in Britain we tend to only use the -ise form.
GB also uses -IZE in most words according to the Oxford Dictionary (but there are exceptions).
As a premium account holder of the Oxford Dictionaries, I have access to more information regarding the words, where it states in -IZE words "also blah blah -ISE".
I wrote an example in the FAQ: http://proofingtoolgui.org/faq.html#11
But pressing the link there will only show the full Oxford description to premium users, persons who are just visitors won't see it. Nothing I can do about it.
I believe that persons from Australia and Canada use -IZE and this speller can also be used by them.
I am planning to create a module for my Proofing Tool GUI tool to allow future AU+CA maintainers to create a list of words to add and to remove in/from the GB speller, so that they can create high quality spellers for their countries, based on mine (it is in my TO-DO list)... what I don't know is who would be interested in such a hard task.