Some time ago I ran into the problem Paul mentioned in his first post.
Now, the problem reoccured. I am not quiet sure which IPFire release caused that first, but I would guess it must have been the latest.
Even putting the Fritzbox’s IP/ adress-range into the “Network based access control” does not solve the problem. Excluding “fritz.box” from Firefox’s proxy settings solves it.
Do you have any idea what causes that trouble again?