Hi, I puzzeled some more and found the solution myself. In Tab Mix Plus I had to change a setting. I will try to explain what I did, but I don't know the English words that Firefox is using, so I will try to translate and hope it looks like the English version of Firefox.
In Firefox go to "Tools" > "Tab Mix Plus options" > "Layout" > "Tab" > "Ask attention for: > then make sure the box before "unloaded tabs" is unchecked.
This was the solution for me...