Add a Dictionary to the Mozilla Thunderbird Spell Check
Your Mozilla Thunderbird’s spell check may be limited to finding fault with your English, regardless of whether you compose your emails in Arabic, English, French or even Icelandic. Given how useful spell checking can be, especially with languages other than the language you grew up speaking, Mozilla Thunderbird should be able to learn new tongues. Fortunately, it can. All it needs is the proper dictionary. To install a new dictionary for the Mozilla Thunderbird spell checker:
Add a Dictionary to the Mozilla Thunderbird Spell Check in Mozilla Thunderbird 2/3
To install a new spell checking dictionary in Mozilla Thunderbird 2 and 3: You can use the new dictionary by selecting it from the Spell drop-down menu (select the down arrow) while composing a message.