There is not really a difference between a dictionary and the spelling checker. The spelling checker needs a dictionary in order to be able to do its job because without a dictionary it is not possible to know whether a word is spelled wrongly.

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 enable or disable spell checking globally:

• Tools > Options > Advanced : General: Browsing: "Check my spelling as I type"

You can look here for dictionaries:

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

You can set the layout.spellcheckDefault pref to 2 on the about:config page to enable the spelling checker for both single-line input fields and multi-line text areas.

• http://kb.mozillazine.org/layout.spellcheckDefault

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

hugovargas said

Please help me with this simple (simple, please) How can I use the add-on dictionaries? I'm not looking for a spell checker, I understand there is a difference between a dictionary and a spell checker. Firefox (withon add-ons) shows a French dictionary available (and a confusing language pack ???) I did install the dictionary, but have no clue how to use it (dictionary, not spell checker) OSX 10.6.8 (Firefox 39.0) Thanks for your help ...

Thanks for the answer. Unfortunately there is something else that prevent my french dictionary to work. I understand that I have to select text first, then right click. When I do right click there is no Language sub menu. Thanks anyway

"Dictionaries" in Firefox are only used for the spellchecker. Not for definitions. I know that is confusing.

Since you wanted a translation/definition of a word, try this addon: https://addons.mozilla.org/en-US/firefox/addon/s3google-translator/

For a actual definition of a word, there is also: https://addons.mozilla.org/en-US/firefox/addon/define-word/