Change the comment span to a div and add an style="overflow:scroll;" or add it to the .comment style. overflow:scroll doesn't work well on spans.
Or put a div inside the comment span containing the comment content with the same attribute, overflow:scroll then it will have a horizontal scroll on that item not the page.
However this might want to be tested as you may need some min and max settings so that long vertical comments don't scroll. Probably not a quick fix the way things are setup.
You will also need to add width tags climbing up the tree to make overflow work. Width: 100% on the tables, tr, and td's (possibly not all of them, I didn't test it fully.)
(And you can't put pre in a span, so change the span to a div like drawkbox said.)
But use overflow: auto not overflow: scroll.
And if you are worried about vertical scroll use x-overflow: auto (doesn't work in opera though).
Or put a div inside the comment span containing the comment content with the same attribute, overflow:scroll then it will have a horizontal scroll on that item not the page.
However this might want to be tested as you may need some min and max settings so that long vertical comments don't scroll. Probably not a quick fix the way things are setup.