This is an issue with fluent that was surfaced by the switch from dtd to fluent in wizard.js . There are only 2 <wizard> consumers in mozilla-central, but there are notably more in comm-central when I last looked. Adding something like:

<linkset>
  <html:link rel="localization" href="toolkit/global/wizard.ftl"/>
</linkset>

should be sufficient to work around this.

Apologies for this happening so late before merge. :-(

Could we also call MozXULElement.insertFTLIfNeeded("toolkit/global/wizard.ftl"); from the wizard Custom Element constructor / connectedCallback?

(In reply to Brian Grinstead [:bgrins] from comment #1)

Could we also call MozXULElement.insertFTLIfNeeded("toolkit/global/wizard.ftl"); from the wizard Custom Element constructor / connectedCallback?

We already do that, it doesn't work, see the discussion in bug 1625938. Hopefully Zibi will fix the underlying problem but it's not clear that'll make 76.

I haven't tested this. On closer inspection though, I think these are the only 2 wizards in mail/ ? The others seem to be in calendar/mailnews/suite. It's unclear to me to what degree those are still maintained; if they are, they'd want similar patches.

Thanks! I'll make an updated patch

calendar is now an integrated part of Thunderbird and mailnews is as well (backend and parts of UI shared with seamonkey).

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/43a129b74441
<wizard> consumers need a linkset element with at least 1 FTL link (e.g. toolkit/global/wizard.ftl) to avoid missing button labels - following bug 1608200. r=darktrojan