Do you have the en-US dictionary installed and selected?

You can see which dictionary is selected if you right-click in a text area and open the Languages submenu.
At least one dictionary should be installed and have a check mark to indicate that it is selected.

• Open the "Add Dictionaries" link to install a dictionary if you do not have one.
• Make sure that [X] "Check Spelling" in the right-click context menu is check-marked.

You can also check the spellchecker.dictionary pref on the about:config page.

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

I clicked that it was useful to me. However, do I have to check this every time I go to write something or will it automatically correct to American English if I click it the one time?

If you select a dictionary then Firefox should remember the choice and you should still have that dictionary selected on a next start

If you do not keep changes after a restart then see:

• http://kb.mozillazine.org/Preferences_not_saved

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes