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I got it. The html was not valid. There was a small problem with the calendar modul (a tag was missing). I can't believe that this was the reason for this phanomenon. I thought the Firefox Browser is more strict than the IE. But the IE failed totally in this case. Amazing. Thanks to all for the advises, especially to martin42 and wishy.

I check this one more time and will post the error in detail because someone else could have an identical problem in the future.

For info.... There's a cool HTML validator extension in Firefox that shows a Green tick or a Red cross in the bottom right-hand corner of the screen - very useful...

I also use the server http://validator.w3.org because I'm new to CSS, and feeling paranoid!