Firefox 3 ∞
The instructions haven't changed..
While keyconfig still exists and sortof works, I've never ever been able to get this to work.
Firefox 2 ∞
user_pref("keyconfig.main.xxx_key__Close Tab & Focus to the left", "!][var tab = gBrowser.mCurrentTab; if(tab.previousSibling) gBrowser.mTabContainer.selectedIndex--; gBrowser.removeTab(tab);");
presumably this is
user.js, except that doesn't work.
This has something to do with keyconfig - but I don't know what that is..