If you use the latest version of Firefox (59 or higher) and you have localized the interface into your language you might find that Firefox reverted back to the Englesh language. This is because Mozilla decided to use a new variable to configure the localisation of Firefox’s interface.
I found that upgrading from Firefox ESR to Firefox 59 everything is configured automatically as long as the ESR version was configured correctly. Problems may arise when installing Firefox 59 on a clean installation or when upgrading from the previous Firefox version.
If you encounter this problem with Firefox 59 you can take these steps to remedy this problem:
Make sure you have firefox and your locale package installed (I use German as an example):
apt install firefox firefox-l10n-de
Type about:config in the location bar (address bar).
Search for: intl.locale
I had to either add the following value value:
Right-Click and add a new string: intl.locale.requested with the value of your locale. E.g.: de-DE
Close and restart FF. The interface should be in your locale.
If not, check at the bottom of the about:support page: “Internationalization & Localization”.
Copy the output in your post on our forum:
This is how a correct German configuration should look like:
Internationalisierung & Lokalisierung
Angeforderte Sprachen: ["de-DE","en-US"]
Verfügbare Sprachen: ["de","en-US"]
Sprachen des Betriebssystems: ["en-US"]