STR: Open https://searchfox.org/ then in one of the two text fields right click and tick Check Spelling on. Now right click again (the Languages menuitem should be shown now) and the errors appear.

Errors: two times
[Exception... "Component returned failure code: 0x80520012 (NS_ERROR_FILE_NOT_FOUND) [nsIXPCComponents_Utils.readUTF8URI]" nsresult: "0x80520012 (NS_ERROR_FILE_NOT_FOUND)" location: "JS frame :: resource://gre/modules/L10nRegistry.jsm :: this.L10nRegistry.loadSync :: line 759" data: no] L10nRegistry.jsm:759

The source is:

• We try to load languageNames.ftl and regionNames.ftl from browser omni.ja, both fail, producing those errors
• then we try from toolkit omni.ja, both succeed.

The source is mozIntl, and the API is going to be replaced with https://github.com/tc39/proposal-intl-displaynames (see bug 1557727).

Until then, we need to handle that in a sync fashion, which is what we do with this API. I'm open to suggestions on how to fetch the files without producing those errors when files are missing.

Can we just catch NS_ERROR_FILE_NOT_FOUND and not log it to the console?

Yup! 🤦

https://hg.mozilla.org/mozilla-central/rev/509cd3b3ec22