Top page margin bug with the latest WordPress 3.1 with Firefox!

Hey,

While I was working on a theme, I upgraded to the 3.1 version,
then all of the sudden I see a 20px white space on the top of the page, and this bug appears only with Firefox.

I tried to look at the header.php and the css files but nothing has changed and I have no clue on how to fix it.

I don’t even know where to look for a solution since the only cause for this is the new version, and I think that downgrading the version isn’t the solution.

Thank you in advance, I’d appreciate a fast comment/solution.