Text areas like the Post a Reply box that I am typing this in is a "multi-line text box".

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:

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