Header text messed up in some browsers

Hello,

in https://www.vuelveconmigoaitalia.com/ I use a child theme from Twenty_ten developed by me. Today I have noticed that with Firefox 8 (and also with Internet Explorer 8) the header text that should be displayed over the header image is moved to the right and therefore not shown completely. In earlier versions of Firefox, and current versions of Chrome, Opera and Safari it is shown perfectly. I don’t know why. Anyone knows? Thanks in advance.

Here is the pice of code of my stylesheet that refers to the header:

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