posted this on March 11, 2010 10:36 AM
Postbox is compatible with most Mozilla Dictionaries, however, Mozilla recently added a new extension type (ironically called "dictionaries") to distinguish between dictionaries and add-ons. The Postbox extension system does not yet know how to handle this new extension type, so dictionaries that have adopted the new extension type are currently incompatible with Postbox.
We are investigating ways to change our extension mechanism to support this new type, but in the meantime, we've modified some of the incompatible dictionaries by changing their extension type back to the value it's always been before:
Download and save the dictionary to your hard drive. (Firefox users, be sure to right-click the download link, then select "Save Link As..." from the contextual menu.)
In Postbox, go to Tools | Add-ons.
Click the Install button and select the file you downloaded and click "OK".
Restart Postbox and then go to Preferences (Tools | Options on Windows) / Composition / Spelling and select your default dictionary.
Information for Mac OS X
As of version 2.1.2, Postbox uses the Mac OS X system dictionary service and the dictionaries installed on your system. To change the dictionary, go to Preferences | Composition | Spelling and select your desired default dictionary. You can also change this from the Spell check toolbar button in the compose window.
Mozilla dictionaries cannot be installed in Postbox for Mac.