now I know that's not the root cause of your situation, and I am seeing the same "shift" so to speak.
If you have the web-developer extension try doing an
Outline > Outline Block Level Elements to see the dimensions of those columns...
depending on how you have written the elements, the float may apply to the elements surrounding it. Hence... if I float something left, the next element after that will want to start at the top right corner (taking margins/padding) into account of that element / image. So I am going to assume that since taking the float out of the equation corrects the problem, that it has to do with the styling of the left / right columns and the interaction between them.
Now as for the background image not displaying if you remove the
float: property, that is something you might want to report to Mozilla for debugging ???
they feedback form is located at
http://hendrix.mozilla.org/
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