Patch localization/en-US/browser/browserSets.ftl in your browser/omni.ja (a .zip with a weird file extension), it contains stuff like: close-shortcut = .key = W

See: https://github.com/SebastianSimon/firefox-omni-tweaks how it can be done.

I do this myself as ctrl-n has to be new tab for me, forever, and firefox broke the old keyconfig extension years and years ago. (I had ctrl-n create tabs with an external window manager back in netscape 4 times and opera (pre-chrome-fork one) after that.)