Preconditions
Have English by default set at OS level and use a non-en-locale(DE, FR, zn-CH, es-ES).
Affected versions
Fx 100.0b2

Affected platforms
Ubuntu 20.04

Steps to reproduce

1. Launch Firefox.
2. Go to App Menu->More tools->Customize toolbar.
3. In another tab, Open about:preferences and go to Languages.
4. Click on "Set Alternatives" and Add a new language.
5. Change the original build-locale language to something else(eg. from Chinese to Deutsch)
6. Go back to Customize Toolbar page and observe the favicons.

Expected result
All item descriptions should be translated.

Actual result
Some items retain the original language.
Regression range
Not Applicable since this is a recent feature.

Attachments
There's a screenshot showing partially the problem.

Notes
The translation is completed after a browser restart.

A workaround is:

• Close the tab
• Re-open the tab
• The strings are now translated.

I'm marking this as S3. The buttons appear to be using Fluent, but aren't switching the text for some reason. They are XUL elements, and I'm not sure why they aren't changing without investigating further. It appears that there is a <label> that the text changes correctly, but it is hidden and not displayed.

Ok, so the translation is using the ::after pseudo-element and the style content: attr(title) to make it appear. The <label> element is being translated, but this translation is not correctly being applied to the title attribute on mutation. This is a bug in CustomizeMode.prototype._onTranslations.