This is rather similar to bug 126646. We produce an off by one pixel rendering when subpixel is on, though flooring it to CSS pixel would defeat the purpose of subpixel positioning the column content.
m_columnWidth = std::max<LayoutUnit>(0, (availWidth - ((m_columnCount - 1) * colGap)) / m_columnCount);
Talk to Hyatt.
Created attachment 221301 [details]
FF(v26) rendering is in-between subpixel off and subpixel on. Prefer current subpixel rendering.