[TYPO3-english] update to typo3 4.3, BE not working

[albert]

I upgraded my 4.2 installation to 4.3. I use PHP 5+. Changed the 
symlinks to typo3source, to index.php, to typo3lib and typo3.
In Firefox 3.5.7 for Ubuntu the loginbox page is badly formatted, 
however allows me to login to the backend. BE is then not working.
In Google Chrome things go fine.
Have spent a few hours to find out what it could be but i'm stuck now.
Can anyone shed some light on this.
Albert