firefox-esr-all-in-one-sidebar software package provides virtual package provided by xul-ext-all-in-one-sidebar, 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-all-in-one-sidebar
firefox-esr-all-in-one-sidebar is installed in your system.
Make ensure the firefox-esr-all-in-one-sidebar package were installed using the commands given below,
$ sudo dpkg-query -l | grep firefox-esr-all-in-one-sidebar*
You will get with firefox-esr-all-in-one-sidebar package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | firefox-esr-all-in-one-sidebar |
| Version | |
| Description | virtual package provided by xul-ext-all-in-one-sidebar |
| Sub Section | Virtual |