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I have the same problem, using Firefox 0.8. It happens with most themes, the center and right blocks content gets pushed to the far right, 80% of it is off-screen and you need to scroll to see it. Refreshing the page fixes it. It affects pretty much all themes really, so I just decided to wait until Firefox releases a final version before worrying about the problem.
vagelis, what version of Firefox are you using?
edit: it happens with this new forum too, unfortunately. I didn't have any troubles with the old one
Maybe it's time to update my Firefox