problem with pressing 'enter' in tinyMCE using firefox on ubuntu

Hi, I hav a problem with tinyMCE.
I used
:convert_newlines_to_brs => true,
:force_br_newlines => true
to add <br/> when user press enter.
This works fine with IE and FireFox (running on window)
but when press enter using FireFox running on ubuntu,
the <br/> is not added.

How can I solve this problem? hav some comments?