Problem with pressing 'enter' in tinyMCE using firefox on ub

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

How can I solve this problem? hav some comments?