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

Solution 1:

In any text box in Firefox, right-click, and you should see a menu item "Languages". Select "English / United States".

Solution 2:

Get any language you like at https://addons.mozilla.org/firefox/language-tools/ and then disable every language you don't like through menu Tools, Add-ons.

Or, if you want to have multiple languages installed: after installing them, right click in any text box and choose sub menu Languages to select your favorite/favourite.

Solution 3:

Just setting the language doesn't appear to solve the issue. Firefox seems to randomly use any one of the installed dictionaries every time it starts. Several English dictionaries get installed by default in Ubuntu. The solution is to remove all but the US English dictionary using apt-get.

From How can I change Firefox's default dictionary?, Insperatus posted this useful command:

sudo apt-get remove myspell-en-au myspell-en-gb myspell-en-za hunspell-en-ca && sudo apt-get install myspell-en-us

Before:

After: