Well it depends on the time you have. I would say go for the Mozilla dictionaries.
What I miss is the possibility to add languages as I wish, with a few clicks, as I do for blogging online with Firefox. Selecting several languages is essential to me.
Those here who are monolingual here can still try and install another dict in Firefox. Then when you write in a forum for instance, you can righ-click to change the spellchecker language.