This is Mozvoikko, an extension for Mozilla programs for using Voikko spell checker.

Adding new languages
====================

This extension may include spelling dictionaries for one or more languages. It is not
possible to install multiple Mozvoikko extensions simultaneously but you can add
dictionaries into an already installed extension by placing the dictionaries in any
of the following locations and restarting Firefox:

- Under the Firefox profile folder:
  Firefox profile folder can be located by following these instructions:
  https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data
  Additional ZHFST speller dictionaries can be placed in the sub-folder
  extensions\{b676e3ff-cda7-4e0c-b2b8-74e4bb40a67a}\voikko\3
  Note that if you remove or re-install your Mozvoikko extension these
  additional dictionaries may need to be installed again.
- On Windows:
  ZHFST speller dictionaries can be placed under c:\voikko\3
- On OS X:
  ZHFST speller dictionaries can be placed under /Library/Spelling/voikko/3
  or similarly named folder under user's home folder.
- On Linux:
  ZHFST speller dictionaries can be placed under /etc/voikko/3