This is obviously a great resource and I thank you for creating it. The reason I'm giving it four stars, however, is that is often misleads me in my work. This is because in some of my work I need to use the conservative British spelling (e.g. legal or academic texts, or just personal preference) and the dictionary not detecting those more 'liberal' spelling variants requires me to triple check everything. This is why I would suggest that you create a branching of this add-on that uses the conservative spelling (I guess others might find it useful, too).
By the way, I just tried manually replacing the AFF and DIC files inside the XPI package but the add-on wasn't detected by Firefox (because of digital signature?), so I can only hope that you will create the conservative variant I need.