This addon gives on the possibility to define custom commands to execute when right clicking on links (that includes audio, video and images). # Install required script Sorry, this is a restriction of webextensions. You can read more here

Install Python 3
Download executor.py(python3 script) to a temporary directory of your choice.
Execute script python executor.py install --copy

When running a firefox fork, you have to find the directory for native messaging yourself! # How to use

Define commands and their parameters in the addon preferences.
Right click a link, audio, video or image
Execute command

# How it works *This is how webextensions work and not my choice*. If you want to complain about it, contact the firefox devs, not me. That executor.py script you downloaded and then "installed" is used by Firefox. The executor also installed a manifest that describes, among other things, where the executor script is and who may execute it (this extension exclusively). The script simply reads the input given to it, parses it to JSON, and calls the command (with is arguments) contained in the JSON. When you right click on a link then click on the command to execute, all lines you configured in the addon preferences for that command are put into a list. If you have $URL anywhere on a line, it's replaced by the URL you right click. That list is then sent to the executor in the correct format (by firefox).