I am using FireFox latest version. And my site using K2 is http://onlinemarketingpro.us and you can login by using test / test as user/password and submit articles to see the following problems.
Problems :
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1. I have seen when anybody add an item in K2 after registering then the pop up screen is partially showed.
Save and Cancel button is almost hidden.
2. After posting a new ITEM the following message is shown.
"You don't have the permission to publish items.
Item Saved"
Then the content is shown in EDITOR. User has chance to republish items many times.