very weird firefox behaviour

[Renee Klawitter <klawitter@email.de>]

Hi!
I recently installed the firefox 1.5, (1.4.99+1.5  to be perfectly 
accurate).
Now, when I try to start firefox via the applications menu or via 
console, logged in as a normal user, it won't start and gives me this 
nice little message instead:

Firefox is already running, but is not responding. To open a new window, 
you must first close the existing Firefox process, or restart your system.

But there is no other firefox process. And the curious thing is, logged 
in as root I can start firefox, and as long as the root firefox process 
is running, I am able to start a new window as a normal user!!

Any suggestions ?
Thanks in advance, Renee!