When Firefox integrated the spelling checker I didn’t immediately realized its value, but using it day after day I became “spell” addicted and now I can’t imagine my browser without this feature.
You can install all dictionaries you need, but many people uses only the dictionary for its own language.
Obviously I’m not a normal person so I use three dictionaries; Italian, English and French switching between them when necessary.
Following Murphy laws every time I start to type inside a input box I discover I’m using the wrong language and I must switch it from context menu, a very annoying practice.
Finally I’ve solved this problem writing the extension SpellBySite.
SpellBySite switches automatically to the correct dictionary looking for it on user settings.
For example user can set English dictionary for mail.google.com, Italian for twitter.com and so on.
When input boxes receive focus the dictionary is set and spell checking executed, no more needs to switching from context menu 
Now my life is a bit easier 
PS
SpellBySite is waiting on AMO to escape from sandbox 
Amazon wish list
