Blog not displaying correctly on larger monitors

Hi there,

My blog is displaying incorrectly on certain monitors.

It displays fine in both IE and Firefox on my computer (1280×800 resolution):
https://img.photobucket.com/albums/v81/Sacco26/Domz/homepage_correct.jpg

But the menu columns and text are misaligned on my dad’s laptop on Firefox, even though IE is fine (1920×1080 resolution):
https://img.photobucket.com/albums/v81/Sacco26/Domz/homepage_incorrect.jpg

I’ve recently applied a new theme to my blog (Copyblogger 3 columns 1.1).

Could anyone please tell me how this could be fixed?

I appreciate any help someone could offer me.

Thanks in advance,
Dom