This is great except doesn't work properly due to the way Firefox handles dictionaries.
See below for instructions how to fix this by Pijuvwy "How to fix it: Making the Australian dictionary work!" to make it work properly. Thank you Pijuvwy !
I just did it for Firefox and Thunderbird and now I not longer have to add Australian words such as "favour" to the dictionary. Plus it now correctly identifies USA versions as incorrect. OMG! But still marks "anymore" as misspelt.
If you now want to remove all those words you had to add to dictionary or have some misspelt works, go here for instructions on how to do this. Have to hunt around a bit though. https://www.computerhope.com/issues/ch001058.htm
This review is for a previous version of the add-on (2.2).