[TYPO3] BE does not work with Firefox 1.5.0.4??

[Andreas Balzer]

Hi!
One of my site-admins is using Firefox 1.5.0.3. He get's a "You must use
4+ browsers with TYPO3!" error when trying to open the typo3 directory
(to login).

His browser agent is Mozilla/5.0 (Windows; U; Windows NT 5.1; de,
rv.1.8.0.4) Gecko/20060508 Firefox/1.5.0.4

He said, that he tried Opera too - without success.

What can he do?
Is there any extension that's causing this?

Help appreciated.
Andreas