How to change the language of the Firefox built in spellcheck from UK English to US English?

Problem

For whatever reason, on my Ubuntu install of Firefox, my spellcheck tells me 'flavor' should be 'flavour' and 'initialize' should be 'initialise', etc.. At first, I thought I was crazy and had been spelling it wrong my whole life, but I suddenly realized (ugh there it goes again!) that the spellche…