What does ^+Z do to a process

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http://askubuntu.com – In command line I wrote: michael@ubuntu:~$ firefox Firefox launched. Command prompt ($) disappeared. Then I pressed ^+z. The result was: [1]+ Stopped firefox Well, Firefox really stopped. It did not react anyhow. So, I could not close it by pressing the cross of the window. By the way, the menu items 'Tools', and 'Help' somehow showed some signs of life. They showed drop-down lists, but nothing more (no reaction to inner menu items). Well, I could not end Firefox through System Monitor. Could you comment on this case: what happened after pressing ^+z? (HowTos)