The webextensions commands documentation states that I can use specific media key strings for specifying command shortcuts. These magic strings aren’t mapped to media keys from what I can tell and the only way I was able to figure out what they are mapped to, was by seeing a console warning message about the ‘r’ and ‘i’ keys.
Key event not available on some keyboard layouts: key=“r” modifiers=“accel,alt” id=“key_toggleReaderMode” browser.xul
Key event not available on some keyboard layouts: key=“i” modifiers=“accel,alt,shift” id=“key_browserToolbox”
It turns out the MediaPlayPause is mapped to ‘r’. ‘i’ doesn’t seem to be mapped to any media key shortcut.