Firefox 'Check Spelling' non-US English fix

This is a silly hint, but it is one of those thing that is big enough to annoy, but small enough not to bother looking for a solution... In Firefox 3.x, the spell checker seems to default to a 'British' like version of the English language. It's actually English from Zimbabwe. Here's how to fix it:

Right click on a 'text box', and then: