I have been through all the steps to change the default dictionary to British, (downloaded the dictionary and language packs, changed the settings in options and removed EN-US, changed the setting in the right click context menu and deleted the EN-US files in the dictionaries folder). However, although words like colour are correctly checked, words that should end in -ised in GB-EN are allowed even when spelt -ized. There may be others as well but I haven't got time to check them all. The right click menu no longer has an English (American) option, only English (United Kingdom), and the Languages section in Options only has EN-GB. How can I remove ALL Americanisation from Firefox?