I have had the same issue. One option is to use the Shortkeys Extension? Have you tried it?

Install the Shortkeys Extension: https://addons.mozilla.org/en-US/firefox/addon/shortkeys/

1. Add the extension to firefox
2. Open the settings.
3. Assign it to any unused or blank action.
4. Save the settings and restart firefox.

Second option is to modify Firefox's userChrome.js and userChrome.css files.

Let me know if this works, or you would like the details about modifying Firefox's files.