firefox-esr-l10n-sr software package provides Serbian language package for Firefox ESR, you can install in your Debian 8 (Jessie) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install firefox-esr-l10n-sr
firefox-esr-l10n-sr is installed in your system.
Make ensure the firefox-esr-l10n-sr package were installed using the commands given below,
$ sudo dpkg-query -l | grep firefox-esr-l10n-sr *
You will get with firefox-esr-l10n-sr package name, version, architecture and description in a table
| Distro | Debian 8 jessie |
| Section | Language packs |
| Package | firefox-esr-l10n-sr |
| Version | 45.5.0esr-1~deb8u1 |
| Description | Serbian language package for Firefox ESR |
| Sub Section | localization |