Hunspell has replaced the old MySpell spell check engine in Thunderbird 3. Hunspell also replaced MySpell in OpenOffice.org last year. Good news is dictionaries that are already installed while for MySpell will work with the Hunspell engine.
Hunspell offers support for Asian languages, Unicode support, morphological analysis and support for complex compounds and agglutinative languages like Azeri, Basque, Estonian and Hungarian.
No word at this time when Hunspell will replace MySpell in Fx 2.0.0.X/3.X or TBird 2.
News Source: Mozilla Links