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 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/

That's it! thank you, that was really simple, its a shame that this is not automatically installed by default, It does not seem to check for grammar, is there a separate add-on for that as well?

You're welcome.

The reason is that most locales other than the en-US locale do not come with a dictionary installed due to licensing reasons.

No, the Firefox spell checker can only check for spelling errors and not for grammar and other errors.
You would have to compose the text in a text editor that supports these features and paste the text in the text area in Firefox.