Only some Firefox locales from Mozilla come with a dictionary due to license reasons. Dictionaries are listed at https://addons.mozilla.org/firefox/language-tools/

It is possible that you confused dictionary with language pack and that you actually do not have a dictionary installed. You may want to use the en-GB dictionary if there isn't a special dictionary for your language. Do you have a dictionary installed and selected? You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu. 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. The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark. You can look here for dictionaries: *https://addons.mozilla.org/firefox/language-tools/ Note that for spell check you need a dictionary. A language pack provides string translations for the user interface. *https://support.mozilla.org/en-US/kb/use-firefox-another-language