How can I change Firefox's default dictionary?

Firefox seems to use myspell/hunspell and listing /usr/share/myspell/dicts/ shows me the dictionaries that I'm given options to use:

$ ls -1 /usr/share/myspell/dicts/*.dic
/usr/share/myspell/dicts/en_CA.dic 
/usr/share/myspell/dicts/en_GB.dic
/usr/share/myspell/dicts/en_ZA.dic
...

Search for the dictionary filename in installed packages:

$ dpkg -S /usr/share/myspell/dicts/en_CA.dic
hunspell-en-ca: /usr/share/myspell/dicts/en_CA.dic

So with the Canadian language package name, the fix is to remove it:

sudo apt-get remove hunspell-en-ca

A simple answer is saying that a language pack is an Add-on.

• Go to Tools -> Add-ons.
  If you are using Ubuntu 12.04 or newer, you can use Dash to search in your menu; Tap Alt, and start typing Add-ons.

• After you open the Add-ons manager, click on the Languages tab in left menu. There, you can disable any language.

• After adding a language, just right click on any text area, or input field (By default input fields have spell checking disabled, so you will need to enable it before proceeding to the next step.).

• Choose Language from the context menu, and select the language you want, and this will become your default language until you change it again.

P.S. Clicking on "Add language" in the context menu, will take you to a page which contains a listing of language dictionary packs that links to their corresponding add-ons.

Insperatus's comment deserves to be a full answer. I've had this problem not only with Canadian spelling, but sometimes Firefox would use UK or Australian spelling. On Ubuntu Linux, removing the dictionaries from the system for non-US English fixes this problem.

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: