United States English as Default Language

I chose English as my default language when installing Ubuntu, but still in some applications, especially in Firefox, the spell-checking would use Great Britain English (insisting that there is a "u" in "color" "favorite" and "behavior"). To change it to US English, simply navigate to a page in Firefox that has a text box (i.e. a web form to send an email or something), right click, choose Languages, and choose "English / United States."