I tried to change the behavior of ctrl-Tab from 'Next Tab' to 'Previous Tab' and added 'alt-Tab' for 'Next Tab' in Keyconfig so I can navigate in both directions with one hand. It does not change the behavior of ctrl-Tab. I created both these shortcuts in Keyconfig: Next Tab gBrowser.mTabContainer.advanceSelectedTab(1,true); Previous Tab gBrowser.mTabContainer.advanceSelectedTab(-1,true); I even tried doing this from the command line with the same result: defaults write org.mozilla.firefox NSUserKeyEquivalents -dict-add "Next Tab" "~Tab" defaults write org.mozilla.firefox NSUserKeyEquivalents -dict-add "Previous Tab" "^Tab" I'm not sure why this didn't work but maybe "Tab" is the wrong keyword. I couldn't find any documentation on what it should be. Or possibly "Next Tab" and "Previous Tab" don't work in this case because they are not 'built-in' commands. A third possibility is that it won't work because ctrl-Tab is a built-in shortcut but I can't believe that is why. I was able to change cmd-Q to ^-cmd-Q no problem and that is a default OS X shortcut. I was also able to change other built-in functions' shortcuts, just not this one. So what am I doing wrong? Has anybody been able to change this particular shortcut?