I think this is problem many have, but i still did not found solution. Here is link to the page where i get the error: demo.xavortm.com (this is my free wordpress theme)

Now, i can fix this if i write

body {height:100%}

But then the problem comes with firefox. It looks like that: http://i.imgur.com/lHpaC.png

You can see everything here: https://github.com/xavortm/Inplus
(the code is little old and body height is still 100% so it all works, just firefiox problem ...

Please help ^^