lower panel launchers in xfce

Hello. I'm wondering if it is possible to change the click property of the lower level/lower panel launchers in xfce4. Specifically, is it possible to make it so that no matter how many times someone clicks on a launcher, that only one instance or window of the program will show up? I ask because recently I had a friend over who wanted to look up something on her facebook page, and I invited her to use my computer. So, she went to the lower panel, and repeatedly clicked the web browser icon, which resulted in numerous instances of firefox being opened, and subsequently caused a freeze in firefox to occur. She stated, "oh, never mind, something's wrong -- I can wait." It drives me crazy when a Windows user perceives a failure in my system, regardless of whether the perception deserves any merit or not. And every Windows user I've ever met just seems compelled to mindlessly keep clicking the icon until the the program opens.