From MozillaZine Knowledge Base

This article is Firefox specific, yet its only category is Configuration. I suggest the article either be modified to support Firefox, Thunderbird and SeaMonkey, or it be explicitly identified as Firefox only and added to the Firefox category. I notice that Dictionaries covers both Firefox and Thunderbird by the way.

There is a obsolete Inline_spellchecking_-_Thunderbird article talking about an extension that is no longer useful now that spell check as you type is built-in. I suggest that article be redirected either to this article (if it also supports Thunderbird) or to a equivalent of this article for Thunderbird. Tanstaafl 11:13, 4 February 2012 (UTC)