In case you want to get rid of the vertical bump, you can try "display: table-cell" instead of inline-block, but you may have to embed the panels by threes in a div. Plus, the browser from Redmond may ignore you depending on the version.
Debian Wheezy's Iceweasel (aka FF 17) on a Toshiba Satellite A215. 1280x800 screen.
I saved a local copy of the page you posted to test things. That may have had something to do with it, although the rest of that page section looked OK. I've shut it down for the night. I'll see if I can reproduce things tomorrow.