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?