i made fixed header and footer and what is strange on some computers in the same version of firefox when i scroll the text, header and footer 'jump'. they are still in the same possition after scrolling but it looks like they change place and then in 0.5 sec. goes back to previous possition.
i had the same issue with firefox.
removing the fixed header improved performance dramatically, the footer alone is almost perfect now.
there's nothing you can do to fix it except wait for firefox...