CSS: series of floated elements without wrapping but rather scrolling horizontally

How about using display: inline-block this way you can use borders on the block elements and get the horizontal scroll bar.

#thumbnails_container {
    height:75px;
    border:1px solid black;
    padding:4px;
    overflow-x:scroll;
    white-space: nowrap
}
.thumbnail {
    border:1px solid black;
    margin-right:4px;
    width:100px; height:75px;
    display: inline-block;
}

Did you try

white-space: nowrap;

in your #thumbnails_container?

Instead of floating, try this:

#thumbnails_container {
    height:75px;
    border:1px solid black;
    padding:4px;
    overflow-x:scroll;
    overflow-y: hidden;
}
.thumbnail {
    border:1px solid black;
    margin-right:4px;
    width:100px; height:75px;
    display: inline;
}

Remove the div { overflow:hidden; }, the rest stays the same.

It's a bit simpler and they'll span across and scroll like you want.