firefox-esr-debianbuttons software package provides virtual package provided by xul-ext-debianbuttons, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install firefox-esr-debianbuttons
firefox-esr-debianbuttons is installed in your system.
Make ensure the firefox-esr-debianbuttons package were installed using the commands given below,
$ sudo dpkg-query -l | grep firefox-esr-debianbuttons*
You will get with firefox-esr-debianbuttons package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | firefox-esr-debianbuttons |
| Version | |
| Description | virtual package provided by xul-ext-debianbuttons |
| Sub Section | Virtual |