Firefox has a built-in spelling checker see

• http://dmcritchie.mvps.org/firefox/tab_capacity/001_with_underscore.htm#spellcheck
• http://dmcritchie.mvps.org/firefox/tab_capacity/001_with_underscore.htm#spellcheck_jumpstart

Once set up, you will see wavy lines under misspelled words. Right-click on the identified word to replace with corrected spelling or to add to the dictionary.

• You can see which dictionary is selected if you right-click in a text area and open the Languages submenu.
• 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 enable or disable spell checking globally:

• Firefox > Preferences > Advanced : General: Browsing: "Check my spelling as I type"

You can look here for dictionaries:

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

See also:

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