The toggledocumentcolors.shortcutKey entry should be set to the alphabetical/numeric character that you want to use. The toggledocumentcolors.shortcutModifiers entry should indicate which other keys to include (accel, shift, alt, etc.) This page lists and explains the possible Modifiers: https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/hotkeys
For example, the default Toggle shortcut key is set to Control+Shift+C: toggledocumentcolors.shortcutKey: C toggledocumentcolors.shortcutModifiers: accel,shift
If you let me know what shortcut key you want to assign and what kind of system you are on, I may be able to provide more specific help. Just make sure that Firefox and your other add-ons aren't using that same key combination for anything else already. I think there is a way to list all the existing shortcuts in use, but I don't remember right now how it is done.
Re: toggle shortcut
This page lists and explains the possible Modifiers:
https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/hotkeys
For example, the default Toggle shortcut key is set to Control+Shift+C:
toggledocumentcolors.shortcutKey: C
toggledocumentcolors.shortcutModifiers: accel,shift
If you let me know what shortcut key you want to assign and what kind of system you are on, I may be able to provide more specific help. Just make sure that Firefox and your other add-ons aren't using that same key combination for anything else already. I think there is a way to list all the existing shortcuts in use, but I don't remember right now how it is done.