text-anchor "middle" and "end" + dx

It appears that Firefox and Opera don't observe the dx attribute on
text elements in conjunction with text-anchor "middle" or "end". For
example, I would expect the dx attribute here to shift the text left
by 10 pixels, and it does on WebKit, but not Firefox:

  <text dx="-10" text-anchor="middle">Hello, world!</text>

This bug was reported more than three years ago, and I haven't seen
any progress on it:

  https://bugzilla.mozilla.org/show_bug.cgi?id=454970

Test case here:

  http://zajec.net/bug/view/svg.text.anchor

It's fairly easy to work-around this behavior by modifying the x
attribute directly, but it's an annoying discrepancy between browsers.
Which behavior is correct?

Mike