It seems that in ''Firefox 3.5.5 (Linux)'' it will render div content on top of PDF plugin if it has overflow:auto (or one of overflow-x or overflow-y set to auto).
Adding this to ''.ui-dialog'' style improves the rendering (border and padding of dialog are still hidden), but it also make dragable dialogs movement erratic.
Solution seems to be that we need to envelop the whole ui-dialog into another div (sizing of which needs to be handled manually, not sure how well moving/resizing would work). This wrapper div need have no style other than overflow:auto, but we must be careful not to actually produce any scrollbars around it
In case of using modal dialogs, then just the overlay needs overflow:auto. Note that this will cause all of PDF plugin to hide when overlay is placed on top of it (as if it's replaced with a transparent div)