• You can see which dictionary is selected if you right-click in a text area and open the Languages submenu.
• 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"

See also:

• http://kb.mozillazine.org/Spell_checking
• http://kb.mozillazine.org/Dictionaries

You can look here for dictionaries:

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

Thank you (Go raibh maith agaibh, tá an fhadhb réitithe anois) the problem is now solved.

You're welcome.

Google translates:

Go raibh maith agaibh, tá an fhadhb réitithe anois:
Thanks, the problem is solved now