For anyone else who is tired of losing unsaved comments after accidentally hitting ^w:
1. Load about:keyboard
2. Find "Close tab" and click "Clear" or "Change".
There doesn't appear to be any other way to directly address this issue within ff, short of modifying the source and recompiling. According to MDN[0], manifest.json does not support changing internally defined keyboard shortcuts:
>If a key combination is already used by the browser (like "Ctrl+P") or by an existing add-on, then you can't override it. You can define it, but your event handler will not be called when the user presses the key combination.