The 'Any', 'Paid', and 'Free' tabs don't align as they should because the 'box-sizing' CSS property isn't supported in Firefox. Firefox requires the vendor prefixed '-moz-box-sizing' CSS property. Perhaps this could be added to the 'xenforo_tabs.css' under the '.tabs' selector.
Better yet, adding this to the preprocessor (similar to border-radius and box-shadow properties) would be helpful for forgetful/lazy people like me!
Obviously, not a big deal. Just thought you should know!