By the way, here is your earlier thread: [https://support.mozilla.org/en-US/questions/930710 When I'm in Windows Live Hotmail and I try to quit Firefox it doesn't work. I have to manually select Quit from the dropdown menu. mac OSX]. There you provided this additional information: ''When I'm in Windows Live Mail and I try to quit Firefox, a dialogue box appears: "You need to select at least one message before you can do this. To select one or more messages, just check the box before each message." I have to manually select Quit from the dropdown menu.'' It sound as though Windows Live Mail is intercepting that key combination for one of its own functions. If Microsoft doesn't let you turn that off, you might be able to override it using an add-on or userscript.