See this answer in another thread: https://support.mozilla.org/en-US/questions/1252712#answer-1204026

Sorry, doesn't help.

What Firefox locale do you use? Only a few locales come with a dictionary due to licensing reasons. For locales that do not come with a dictionary you need to install the dictionary yourself. If you do not have the en-US locale then check that you have a dictionary installed (Tools > Add-ons > Dictionaries) and enabled. 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 enable or disable spell checking globally: *Options/Preferences -> General: Browsing: "Check my spelling as I type" You can look here for dictionaries: *https://addons.mozilla.org/firefox/language-tools/ Note that you need a Dictionary and not a Language Pack that provides string translations for the user interface. You can check the layout.spellcheckDefault pref on the <b>about:config</b> page. *http://kb.mozillazine.org/layout.spellcheckDefault You can open the <b>about:config</b> page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue. *http://kb.mozillazine.org/about:config The web page can disable the spelling checker via a spellcheck="false" attribute of this input or textarea or editable element and possibly provide its own spelling checker. Firefox will disable Check Spelling in the right-click context menu in this case. You can right-click in the text editor area and use "Inspect Element" to see if this text area has a spellcheck="false" attribute.

When I right click in arext area the drop down menu does '''NOT ''' have an option for languages.