I figured out what the source of my javascript problem was, maybe soneone has this to, so I'll post the solution here.
PROBLEM: Login in with firefox always fails with a javascript error.
Error: missing ) after condition Source File: ../js/app.js Line: 2212, Column: 6 Source Code: this.request_obj = new NULL
I fixed this one with adding activeX = window.ActiveXObject;
in the first line of the this.http_request function (around line 2200 in app.js)
and then changed else if (window.ActiveXObject) to: else if (activeX) a few lines later
After that, roundcube-login worked well (With IE 6 && FF 1.0.7)!
SOLUTION: At my office, I'm sittin behind a very strict firewall(proxyserver), which deletes all occurences of potentially dangerous things like "window.ActiveXObject" out of the source code. That's all. I you can implement my workaround as posted above, I and maybe others won't have any more troubles with tight proxies in the future.
Thanks, Patrick
VHCS Webmail