alt131 - 1:05 pm on Jan 1, 2012 (gmt 0)
Hi Paul. beat me to it ;) What is interesting is that it was first noted when the recommendations didn't specify where the scrollbar should be placed, but it has not been re-visited now they do. I'm guessing the bug is the reason box-sizing isn't an effective cure as well.
Another bug is that firefox doesn't recognise % padding on textarea, which can be used to overcome this, but unfortunately ff interprets a % length for one side as padding: 0 for all sides which defeats the purpose here.
An additional issue is that ff displays the resize handle the same colour as the textarea background which creates an even larger bottom "padding". (Unlike winsafari, for eg, which clearly makes the resize an extension of the scoll bar.)
So rainlover that makes workarounds awkward, so probably important to identify your priorities. For example, if consistent display is critical, then remove the padding and set an 8px border to re-produce the scroll bar position cross-browser. If the key is to avoid the unsightly top/right/bottom padding then just use padding-left, or perhaps a % padding for the other browsers and a border-left to create the visual space. Line-height can be used to create a visual gap between the top edge and the text. If the text was only going to be a single line text-indent could be used. (Unfortunately ff doesn't recognise first-line on textarea which would provide a more targeted work-around. )