Oxford English Spellchecker — shill

A spellchecker that adheres to Oxford spelling, a variant of British English which uses -ize endings instead of -ise that is used internationally, as well as in the Oxford English Dictionary.

To use this addon:
1. Go into settings and under 'Language' turn on 'Check spelling as you type'
2. When inside of a text box, right click, and verify that 'Check Spelling' is enabled
3. Make sure that 'English (United Kingdom, oxendict)' is checked

This spelling standard is identified by the IETF language tags en-GB-oxendict and en-GB-oed. For more information, see this Wikipedia article.