Localizing Addons

Localizing addons is easy.

Adding messages

Under addons-l10n/en/, make a file named ADDONID.json, where ADDONID is the addon’s ID (the folder name). Write your messages that you want to translate there:

  "ADDONID/meow": "Meow",
  "ADDONID/cats": "{number, plural, one {1 cat} other {# cats}}",
  "ADDONID/eat": "I want to eat {food}!",
  "ADDONID/salmon": "salmon",
  "ADDONID/sardine": "sardine"


Sometimes messages need to have things that are dynamically generated. For example, number of cats, or input. To handle this, you can use placeholders like {placeholderName}. Placeholder name should only contain letters (no numbers).


What if the placeholder is a number? We can use plurals like {placeholderName, plural, one {when there is one thing} other {when there are # things}}. If the placeholder is 1, it will show “when there is one thing”, otherwise it says “when there are (placeholder) things”.

Using the translations

Change your userscript’s first line from something like:

export default async function ({ addon, console }) {


export default async function ({ addon, console, msg }) {

The msg added is the function you use to get translations. For example, text = "Meow" can now be text = msg("meow"). The addon ID and the slash is omitted.


You can provide placeholder values:

cat = msg("cats", {number: 1}) // shows "1 cat"
cats = msg("cats", {number: 3}) // shows "3 cats"
hungry = msg("eat", {food: "cod"}) // shows "I want to eat cod!"

You can also “nest” messages:

hungry2 = msg("eat", {food: msg("salmon")}) // shows "I want to eat salmon!"


If you are writing raw HTML, msg should be replaced with safer version of safeMsg.