Re-posted here as per Methyl's suggestion.
iMac OS X Yosemite 10.10 Using Firefox,
Loads of space either side AB page, but missing a bit in the middle.
There is a a setting in older versions of FF which let you "set" a zoom level - which sometimes breaks the CSS on some pages. It looks like that to me?
I'm viewing AB using FF v33.1 on a Quad Core i7 Mac mini with no display problems. The only difference is I'm still on Mavericks (v10.9.5) so perhaps the problems is with Yosemite.
I have just had a look using Safari, and it's fine.
It's also fine on my laptop, and on my older iMac which is loaded with OSX 10.6.8 - so perhaps the issue is between Yosemite and AB, everywhere else is fine.