From MozillaZine Knowledge Base
This article is about dictionaries used to check spelling in Firefox and Thunderbird. It applies to releases of these applications starting from 2.0. For information about dictionaries for Thunderbird releases before 2.0, see: Obsolete Dictionaries - Thunderbird
Installing a dictionary
Add-on (extension) dictionaries are easier to install than other dictionaries. Dictionaries are available from these locations:
Installing directly via Add-ons
Installing via Tools | Add-ons is the easiest method, but only works for add-on dictionaries. Use this method to install a dictionary in the current profile. To install the dictionary in another profile, switch to that profile and repeat the procedure:
On the web page, do not click the link that points to the dictionary.
Installing by downloading manually
This method only works for add-on dictionaries. Use it when you want to download the dictionary just once and install it in more than one profile.
To install the dictionary in another profile, switch to that profile and repeat the procedure from step 2.
Installing by using an extension
This method is only for Thunderbird and works for all dictionaries. Use it to install a dictionary that is not packaged as an add-on, or when you want to make the dictionary available in all profiles.
First install the Dictionary Fix extension. To install dictionaries, follow the instructions on that linked page.
Note: You can use other extensions that work with add-ons to install add-on dictionaries in both Firefox and Thunderbird—for example: MR Tech's Local Install
Installing manually to the application
This method works for all dictionaries. Use it to install a dictionary that is not packaged as an add-on, or when you want to make the dictionary available in all profiles.
This method installs the dictionary in the application itself, making it available in all profiles (but see the note below).
At step 5 you usually need a program to open the downloaded file. You can use any zip tool or jar tool—for example: 7-Zip
Note: To install a dictionary manually in a particular profile, at step 3 go to the profile directory. Then at step 4, create the dictionaries directory, if necessary, at the same level as the extensions directory. You might want to do this, for example, if the profile is shared by more than one installation of Thunderbird.
Uninstalling a dictionary
To uninstall a dictionary that you installed as an add-on:
To uninstall a dictionary that you installed manually:
Selecting an installed dictionary
Related preference: Spellchecker.dictionary
After you install a new dictionary, you need to select that dictionary if you want to use it. You can do that by opening the context (right-click) menu of a text area, selecting "Languages" and choosing the dictionary you would like to use. The "Languages" sub-menu shows all installed dictionaries and includes an item ("Add dictionaries") to install new dictionaries from addons.mozilla.org. The context menu also has an item ("Spell check this field") that shows if spell checking is enabled. The Dictionary Switcher add-on adds a toolbar button that can also be used to switch the spellchecker dictionary.
Spell checking can be enabled or disabled using the checkbox, "Check my spelling as I type" in "Tools → Options → Advanced → General / Browsing". You can change the preference layout.spellcheckDefault in about:config directly to enable spell checking for single-line and multi-line text fields.
You can choose the spelling checker’s language in: “Tools → Options → Composition → Spelling → Language”. You can also choose it in the Compose window using either : “Options → Check Spelling... → Language” or by pressing the Spelling button and choosing a dictionary from the list box.
The Markov dictionary switcher add-on detects the language of the message that you are composing after you type four or five words and changes the spelling dictionary. The Automatic dictionary switching add-on remembers the language the spellchecker used when composing a message to somebody and automatically switches to it every time you write to that person.
Spelling errors / Dictionary changes
The contents of dictionaries are not maintained by Mozilla. In some cases they are not maintained by anyone. If you think the contents of a dictionary should be updated, you might be able to find out who maintains it by looking in its README file (if it has one). You can find the README file by using a zip tool or jar tool to open the dictionary's installation file. .xpi files can be opened in windows explorer by first changing the .xpi extension to .zip
Until a spelling error is fixed by the maintainer, you can add what you consider to be valid words to your personal dictionary.
The Auto Correct add-on autocorrects words and automatically capitalizes the first word of a sentence. It has its own autocorrect list which you can edit.
Your personal dictionary is stored in the file persdict.dat in your profile folder. It is simply a list of words in plain text format that you added via the spell checker or by importing. To delete custom entries added to the spell check dictionary, you can manually edit the file persdict.dat in your profile folder.
You can import a list of words from somewhere else by replacing this file, or you can copy this file to use it in a different profile.
Note: In Thunderbird 2, the Dictionary Fix extension automatically fixes some common problems with dictionaries.
Less common problems: