How can I force a CSS column not to move when I crunch my browser?

I’m working with the Gila theme and have manipulated a lot.

But my right-hand column moves when I crunch my browser down in size. It overlaps my center column in Firefox and pushes the center column to the bottom of the page in IE.

How can I force the right-hand column to stay in place so it won’t overlap the center column or push it down when I crunch down my browser window to a different size?

Basically, I want all three columns fixed, whereas currently the third column “sticks” to the right side of my browser.

You can see it happening at my site here: https://www.thechristianguitarist.com

Any help is appreciated!