Hi,
I have tried the joomla login module under wap on firefox and WinWap for windows and both display correctly.
You said that Opera mini on the same phone is OK, so it would seem that the Symbian Browser either gets a different set of instructions, or it can't deal with the instructions that the Opera executes ok.
If you go to
www.mobilejoomla.com/ua.php it will tell you what the phone is giving to Mobile Joomla for MJ to serve the template. Please post that string on this thread so I can see what it is for opera and symbian.
I found that link at
www.mobilejoomla.com/documentation/12-getting-started/50-help-and-problems.html
Do you know of an emulator of the symbian browser that I could run under firefox? I could then see what code causes it to break.