I got your back.
I've had this problem with just about every fresh install I've done of Linux Mint since Mint 12. Here's the solution. Open a browser window and type "about:config" in the address bar. Firefox will try to scare you away by telling you that messing around with these settings will void your warranty. Ignore it and soldier on (Warranty!? What warranty!?). You need to search for and change the following values:
browser.showQuitWarning this is set to "true." Right click and toggle it to "false"
browser.tabs.warnOnClose this is set to "true." Right click and toggle it to "false."
browswer.warnOnQuit this is set to "true." Right click and toggle it to "false."
Rest like a god!
