The freeze happens to the last window that is to be minimized. For example if you have 2 open apps. If you minimize the first one, no freeze will occur. But when you minimize the second one the freeze will occur.
When I switch to another workspace and then come back, the freeze is gone.
This problem is not with Firefox only. It can be with any window. This is a possible duplicate with this one: /bugs.launchpad .net/ubuntu/ +bug/1171451
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The freeze happens to the last window that is to be minimized. For example if you have 2 open apps. If you minimize the first one, no freeze will occur. But when you minimize the second one the freeze will occur.
When I switch to another workspace and then come back, the freeze is gone.