I recently tried out an addon called "keyconfig", and it is ideal for what I want.
It re-binds Firefox's shortcut-keys.
I've not found any other way to do this.
This addon has a public forum of 1739 posts on forums.mozillazine.org
This seems to be a well known add-on, and it worked for me in an Ubuntu VM.
And I almost forgot... Is there any other way to customize Firefox' keyboard-shortcuts?