Thanks for all the suggestions. I must admit I still haven't found the perfect solution yet. :-(
Right now I have made a hotfix by setting the height to fixed. It's not pretty, but it's the best cross browser solution.
My thoughts are that it's maybe the best solution to make a JavaScript that finds all columns with the same top value, which means they are on the same row. Determine the tallest one and set all with that top value to that height. This has to be done on every resize and might give issues?