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# Managing the built-in en-US dictionary
The en-US build of Firefox includes a built-in Hunspell dictionary based on the
[SCOWL] dataset. This document describes the process to add new words to the
dictionary, or update it to the current upstream version.
For more information about Hunspell or the affix file format, you can check
[the Ubuntu man page for hunspell](https://manpages.ubuntu.com/manpages/bionic/man5/hunspell.5.html).
## Requesting to add new words to the en-US dictionary
If you’d like to add new words to the dictionary, you can add your request to
- Include all possible forms, e.g. plural and genitive forms for nouns,
different tenses for verbs.
- Try to provide information on the terms you want to add, in particular
references to external sources that confirm the usage of the term (e.g.
Merriam-Webster or Oxford online dictionaries).
:::{note}
If you’re fixing the existing bug with pending requests, make sure to [file a
new bug][file a new bug] and move the alias `enus-dictionary` (in the *Details* section)
from the old bug to the new one.
:::
## Adding new words to the en-US dictionary
This section describes the process for adding new words to the dictionary:
1. Get a clone of the [firefox repository] (see {ref}`Firefox Contributors' Quick
Reference`), if you don’t already have one, and make sure you can build it
successfully.
2. Move in the dictionary sources directory using this command:
`cd extensions/spellcheck/locales/en-US/hunspell/dictionary-sources`.
3. Identify the current version of SCOWL by checking the file
`README_en_US.txt`: near the top of the file there is a line similar to
`Version rel-2026.02.25`, where `rel-2026.02.25` is the upstream SCOWL
release tag.
4. Clone the upstream SCOWL repository into the working directory and check out
the tag matching that version:
```sh
cd scowl
git checkout rel-YYYY.MM.DD
cd ..
```
SCOWLv2 requires Python 3.7+ and SQLite 3.33+.
5. There’s a special script used for editing dictionaries. The script
only works if you have the environment variable `EDITOR` set to the
executable of an editor program; if you don’t have it set, you can use
`EDITOR=vim sh edit-dictionary.sh` to edit using `vim` (or you can
substitute it with another editor), or you can just type
`sh edit-dictionary.sh` if you have an `EDITOR` already specified.
Copy and paste the full list of words, then save and quit the editor. It’s
not necessary to put the words in alphabetical order, as it will be corrected
by the script.
Note: you might need to install `aspell` on your system (e.g. via
`brew install aspell` on macOS).
6. Run the script `sh make-new-dict.sh` to generate a new dictionary and make
sure it runs without errors. For more details on this script, see the
[make-new-dict.sh] section.
7. Run `sh verify-new-dict.sh` to sanity-check the regenerated dictionary
(see the [verify-new-dict.sh] section). The script must report
`Errors: 0` before proceeding.
8. If everything looks correct, use `sh install-new-dict.sh` to copy the
generated file in the right position.
9. Build Firefox and test your updated dictionary. Once you’re
satisfied, use the process described in {ref}`write-a-patch` to create a
patch.
Note that the update script will modify 2 versions of the dictionary, and both
need to be committed:
- `en-US.dic`: the dictionary actually shipping in the build, it uses
ISO-8859-1 encoding.
- `utf8/en-US.dic`: a version of the same dictionary with UTF-8 encoding. This
is used to work around issues with Phabricator, and it allows to display
actual changes in the diff.
## Exclude words from suggestions
It’s possible to completely exclude words from suggested alternatives by adding
an affix rule `!` at the end of the definition in the `.dic` file. For
example:
- `bum` would be changed to `bum/!` (note the additional forward slash).
- `bum/MS` would be changed to `bum/MS!`.
In order to exclude a word from suggestions, follow the instructions available
in [Adding new words to the en-US dictionary]. Instead of running the
`edit-dictionary.sh` script (point 5), use a text editor to edit the file
`en-US.dic` directly, then proceed with the remaining instructions.
:::{warning}
Make sure to open `en-US.dic` with the correct encoding. For example, Visual
Studio Code will try to open it as `UTF-8`, and it needs to be reopened with
encoding `Western (ISO 8859-1)`.
:::
## Upgrading dictionary to a new upstream version of SCOWL
The English dictionary available in the [firefox repository] is based on the
[SCOWL] dictionary. Some scripts distributed with the SCOWL package are
used to generate the files for the en-US dictionary.
The working directory for this process is
`extensions/spellcheck/locales/en-US/hunspell/dictionary-sources`.
1. Clone the upstream SCOWL repository into the working directory and check out
the desired release tag (e.g. `rel-2026.02.25`):
```sh
cd scowl
git checkout rel-2026.02.25
cd ..
```
SCOWLv2 requires Python 3.7+ and SQLite 3.33+.
2. Run the script `sh make-new-dict.sh` to generate a new dictionary and make
sure it runs without errors. For more details on this script, see the
[make-new-dict.sh] section.
3. Run `sh verify-new-dict.sh` to sanity-check the regenerated dictionary
(see the [verify-new-dict.sh] section). The script must report
`Errors: 0` before proceeding.
4. If everything looks correct, use `sh install-new-dict.sh` to copy the
generated file in the right position and use the process described in
{ref}`write-a-patch` to create a patch.
## Info about the file structure
### mozilla-specific.txt
This file contains Mozilla-specific words that should not be submitted
Note that the file `5-mozilla-specific.txt` is generated by expanding
`mozilla-specific.txt` and should not be edited directly.
### utf8 folder
`dictionary-sources/utf8` is used to store a copy with UTF-8 encoding of the
dictionary files. This is used to work around limitations in Phabricator, which
treats ISO-8859-1 files as binary and won’t display a diff when updating them.
## Info about the included scripts
### make-new-dict.sh
The dictionary upgrade script `make-new-dict.sh` works by expanding (i.e.
“unmunching”) the affix compression dictionaries to create wordlists and
using those to generate a new dictionary.
The upgrade script expects the current upstream version to be kept in the
directory `orig`. On first run (or whenever `scowl/scowl.db` has been
removed) the script will run `make scowl.db` inside `scowl/` to build the
SCOWLv2 SQLite database that `mk-list` reads from.
The script writes intermediate files to `dictionary-sources/support_files/`:
- `0-special.txt` contains numbers and ordinals expanded from SCOWL
`en.dic.supp`.
- `1-base.txt` contains words expanded from `en_US-custom.dic` in the
**previous** version of SCOWL (from the `orig` folder).
- `2-mozilla-nosug-munched.txt` contains the suggestion exclusions (lines
ending in `!`) from the current Mozilla dictionary, kept in their
compressed (munched) form so they can be appended back at the end.
- `2-mozilla-nosug.txt` is the expanded form of the same suggestion
exclusions, used later to filter the `5-*` files.
- `2-mozilla.txt` contains words expanded from the current Mozilla
dictionary, with the suggestion exclusions stripped out first.
- `3-upstream.txt` contains words expanded from `en_US-custom.dic` in the
**new** version of SCOWL (regenerated under `scowl/speller/` by
`make-hunspell-dict`).
- `2-mozilla-removed.txt` contains words that are only available in the SCOWL
dictionary, i.e. removed by Mozilla.
- `2-mozilla-added.txt` contains words that are only available in the current
Mozilla dictionary, i.e. added by Mozilla.
- `4-patched.txt` contains words from the new SCOWL dictionary
(`3-upstream.txt`), with words from (`2-mozilla-removed.txt`) removed and
words (`2-mozilla-added.txt`) added.
In addition, the script writes three files directly to the working directory
(`dictionary-sources/`) so they can be inspected and committed alongside the
generated dictionary:
- `5-mozilla-specific.txt` is expanded from `mozilla-specific.txt` using
the current affix rules from the Mozilla dictionary.
- `5-mozilla-removed.txt` and `5-mozilla-added.txt` contain words that are
respectively removed and added by Mozilla compared to the **new** SCOWL
version. These files could be used to submit upstream changes, but words
included in `5-mozilla-specific.txt` should be removed from this list.
The new dictionary is available as `en_US-mozilla.dic` and should be copied
over using the `install-new-dict.sh` script.
### verify-new-dict.sh
This script runs sanity checks on the dictionary produced by
`make-new-dict.sh` and must be run before `install-new-dict.sh`. It reports:
- Whether the regenerated `.dic` passes the ISO-8859-1 round-trip that
`install-new-dict.sh` performs (a failure here means upstream introduced
characters that the legacy ISO-8859-1 path can't represent).
- Whether each entry from `mozilla-specific.txt` is still present in the
regenerated dictionary.
- Whether every previous suggestion exclusion (line ending in `!`) is
preserved, and lists any new exclusions introduced upstream.
- The line-count delta against the previous shipped dictionary (producing a
warning above a 25% change).
- That the upstream `en_US.txt` from the [wordlist-diff] mirror (fetched
via `curl` from raw.githubusercontent.com at the same release tag as
`scowl/`) is a subset of the regenerated wordlist, ignoring words that
Mozilla intentionally removed (`5-mozilla-removed.txt`). The Mozilla
dictionary should equal upstream `en_US.txt` minus Mozilla removals,
plus Mozilla additions, variants and accented words. The check is skipped
(with a warning) when offline, when `curl` is unavailable, or when the
local `scowl/` checkout is not on a tagged release.
The script exits with non-zero if any check fails.
### install-new-dict.sh
The script:
- Creates a copy of `orig` as `support_files/orig-bk` and copies the new
upstream version to `orig`.
- Copies the existing Mozilla dictionary in `support_files/mozilla-bk`.
- Converts the dictionary (.dic) generated by `make-new-dict.sh` from UTF-8 to
ISO-8859-1 and moves it to the parent folder.
- Saves the SCOWLv2 affix file verbatim to `utf8/en-US-utf8.aff` (the
UTF-8 mirror) and then runs `convert-aff.py` (see below) to rewrite
the affix file in the Mozilla-shipped ISO-8859-1 form.
### Python helpers
A couple of small Python scripts live alongside the shell scripts and handle
encoding-sensitive transformations that are awkward to express portably in
shell. They depend only on the Python standard library and target Python 3.7+,
which is already required by SCOWLv2 itself.
- `assemble-dic.py` is invoked at the end of `make-new-dict.sh`. It takes
the freshly generated UTF-8 `en_US-mozilla.dic` and the munched
suggestion-exclusion list (`support_files/2-mozilla-nosug-munched.txt`,
ISO-8859-1) and rewrites the dictionary with both lists merged, sorted, and
an updated count line on top.
- `convert-aff.py` is invoked from `install-new-dict.sh` to convert the
SCOWLv2 UTF-8 affix file to the shipped `en-US.aff`. It strips
`ICONV` rules, rewrites `SET UTF-8` as `SET ISO8859-1`, drops the
curly apostrophe (U+2019) that SCOWLv2 adds to `WORDCHARS` (since
ISO-8859-1 can't represent it), and writes the result back as
ISO-8859-1. The UTF-8 mirror in `utf8/en-US-utf8.aff` is a verbatim copy
of the SCOWLv2 output and does not go through this script.