You can uninstall or removes an installed firefox-esr-l10n-si package itself from Debian 7 (Wheezy) through the terminal,
$ sudo apt-get remove firefox-esr-l10n-si
If you would like to remove firefox-esr-l10n-si and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove firefox-esr-l10n-si
If you use with purge options to firefox-esr-l10n-si package all the configuration and dependent packages will be removed.
$ sudo apt-get purge firefox-esr-l10n-si
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove firefox-esr-l10n-si