+5  A: 

its your overflow style. change to overflow hidden to remove the scrollbars.

Josh

Josh
Chris Coyier has a fantastic post about the css overflow property a little while ago: http://css-tricks.com/the-css-overflow-property/.
patricksweeney
I know that overflow is helping cause the problem, but I *need* overflow:auto on this. Is a horizontal scrollbar supposed to be an unavoidable side effect?
mike
what your need for overflow and perhaps i can help suggest an alternative.
Josh
+1  A: 

I would file a bug report. It doesn't render that way on my version (for Windows).

StriplingWarrior
Yeah, according to the CSS spec, auto should only show scroll bars when necessary.http://www.w3.org/TR/css3-box/#overflow1
p5ycho_p3nguin
Here's an existing bug report on the topic: https://bugzilla.mozilla.org/show_bug.cgi?id=397367
p5ycho_p3nguin
PP: You should submit that as an answer -- I'll accept it!
mike
Sure thing. :)
p5ycho_p3nguin
A: 

I've just tested this on a windows and mac machine - both are fine, so it might be worth submitting a bug report to Mozilla

slightlymore
A: 

Could it be that your div happens to have width:100% and therefore overflows when you add border?

ilya n.
+3  A: 

There shouldn't be scroll bars for overflow:auto unless the content is actually going beyond the edges of the containing element. This is a bug in Linux versions of Firefox. See the following bug report.

p5ycho_p3nguin