CSS won't cause a problem like that. That's one of it's strengths and weaknesses, it affects your webpage as it's pulled up. Only javascript can affect the behavior of your page. Well... pretty much ;)
I would bet dollars to doughnuts that Firefox is having trouble with your onfocus event. Try calling the event a different way. I think a pretty standard way would be to include name attributes for your text input and your form and try an unobtrusive function, something like...
document.formName.inputName.value = '';
I wouldn't change much. If it's working in the other browsers, you're really close.
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