We're working on a similar fix for thunderbird as well.This Firefox update provides a semi-automatic activation and enabling of the users locales for the Firefox user-interfaces for new and existing Firefox profiles. A user needs only to install the firefox-language pack from within MX Package Installer (MXPI) to get the UI automatically switched to the user locale. In rar situations it might be required for users with existing profile to disable and enable the language-addon within Firefox. Such race conditions might exists either due to well known MMCM (Mozilla Magic Cache Mystic) or due to the secretly known MMPC (Mozilla Moon Phase Conditions).
Thanks Fehlix!

- mike