I've been playing with the newly released Firefox 2.0 since it came out. It worked out of the box, I have some issues with it, sometime it freezes, that might be due to the fact that I use chatzilla, or that I use some flash IM/Jabber software on a dating site. So far so good, the web applications I work with daily did not break.
I do miss something and a quick Google search did not help : I use both French and English as my default languages , and might soon add dutch. How do I add dictionaries to the spell checker ? and how do I tell Firefox which language the inline spell checker should use ?

Here's a Dutch dictionary
https://addons.mozilla.org/firefox/3291/
Et voilà, un dictionnaire en français:
https://addons.mozilla.org/firefox/3445/
Dictionaries need to be installed as an extension. There are already 37 (none of them french unfortunately) of them over at addons.mozilla.org:
https://addons.mozilla.org/search.php?cat=68&app=firefox
To change dictionary, you just right click inside an open textarea and select "Languages"