TYPO3 Core

Did you experience any JavaScript errors on that behaviour?
Do you have additional extensions installed that take care about encryption of the frontend user password?

I have no errors in the frontend and no extensions to encrypt the password

The problem is unfortunately still

I tried logging in with FF 3.5.2 on TYPO3 4.2.8 with felogin and it works without problems.

Try deleting your browser-cache, clear all cache in TYPO3 and delete typo3conf/temp_CACHED.....php.

What exactly happens, if you try to login?

Ok it works. Thanks
For people with the same problem: Clear your Cookies

Thanks for feedback, Felix.

Resolved, no change required.