Firefox Spell Check: Languages, Custom Dictionaries, and Configuration

Firefox has a built-in spell checker for text you type in browser form fields, comment boxes, and other editable areas. It's more capable than most people use.

How Spell Check Works

Firefox checks spelling in textarea elements and contenteditable elements (like WYSIWYG editors). It underlines misspelled words with a red dotted underline.

Right-click a misspelled word to see:

• Suggestions for corrections
• "Add to Dictionary" (adds the word to your personal dictionary)
• "Ignore" (for this session only)

Enabling/Disabling Spell Check

Per-field toggle: Right-click inside any text field → "Check Spelling" checkbox.

Global toggle: Settings → General → Language → find the spell check section. Or about:config → layout.spellcheckDefault:

• 0: Disabled
• 1: Single-line text fields only
• 2: Multi-line text fields (default)

Languages

Installing Additional Language Dictionaries

Firefox comes with a spell check dictionary for the language it was installed in. To add more:

1. Go to about:preferences#general
2. Click "Check your spelling as you type" → click "Manage dictionaries..."
3. AMO opens showing available language dictionaries
4. Install the language you want

Alternatively: search "spell check dictionary" on addons.mozilla.org and install the language pack for your language.

Dictionary Files Location

Language dictionaries are stored as .dic and .aff files in your Firefox profile:

• Linux: ~/.mozilla/firefox/[profile]/dictionaries/
• macOS: ~/Library/Application Support/Firefox/Profiles/[profile]/dictionaries/
• Windows: %AppData%\Mozilla\Firefox\Profiles\[profile]\dictionaries\

You can add custom .dic/.aff files here directly (Hunspell format).

Switching Languages While Typing

In a text area: right-click → "Languages" → select the language for this field. Firefox temporarily switches the spell check dictionary for that field.

This is useful for writing in a different language than your UI language without changing settings.

Custom Dictionary

Words you add via "Add to Dictionary" go to a personal dictionary file:

• Location: same dictionaries folder as above, usually persdict.dat
• Format: plain text, one word per line

You can edit this file directly to add/remove words. Useful for adding domain-specific terms (programming terms, product names, etc.) in bulk.

Technical Terms and Code

Spell check in coding contexts (documentation, comments in online editors) is often noisy. Options:

• Disable spell check for specific sites: right-click → Check Spelling → uncheck
• Add common programming terms to your custom dictionary
• Use about:config → layout.spellcheckDefault = 0 to disable globally and only enable on specific sites via right-click

Grammar Check

Firefox's built-in spell checker handles spelling only — no grammar checking. For grammar, you'd need a browser extension (Grammarly, LanguageTool, etc.) or rely on the web app's own grammar tools.

What Doesn't Get Spell-Checked

• Regular (non-editable) page text — Firefox only checks text you're typing
• Password fields — never checked (by design, for privacy)
• Address bar — not checked
• Single-line inputs with spellcheck="false" attribute — sites can opt-out
• Some rich text editors that handle their own input (CodeMirror, Monaco, etc.)

Linux Specifics

On Linux, Firefox uses the Hunspell library for spell checking. Some distributions package language dictionaries separately:

# Debian/Ubuntu
sudo apt-get install hunspell-de-de  # German example
sudo apt-get install myspell-es      # Spanish example

# Fedora
sudo dnf install hunspell-de

Firefox may automatically detect system-installed Hunspell dictionaries, depending on the distro.

Autocomplete vs Spell Check

These are different features:

• Spell check: Underlines typed words that don't match the dictionary
• Autocomplete: Suggestions from browser history, bookmarks, form history

Autocomplete is handled separately (Settings → Privacy & Security → History → Remember search and form history).

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The per-language right-click toggle is the most practical feature here — switching dictionary temporarily for a field in another language takes one second and works well for multilingual writers.