If the text size in firefox is larger than the default size, this breaks the sites formating and overlaps the right hand side of the site on top of the left.

To cure this, change the text size in the browser to normal.

If you are also using Microsoft Media Center together with firefox, you will get the broken display on normal text size, this is because Media Center enlarges the text ready to be displayed on a TV.

To cure this reduce the size of the text display in your browser 1 setting below normal.

The cause of this is Firefox not enlaging the site restraints along with the text so the text breaks out of its restraints, this should be fixed in future versions of Firefox