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