Neutral directional characters in Mozilla/ Firefox

Mozilla or Firefox for Linux display the following HTML extract (with
the capital letters indicating Arabic text):

<div dir="rtl">
The date is:<br>
2006 YLUJ 17
</div>

... like this:

The date is:
2006 YLUJ 17

Yet, I understand from the litterature on bidirectional Unicode text
that neutral directional characters (like the colon) surrounded by
strong directional characters of different directions (in this case:
the "s" and the "A") receive the same direction as the paragraph (in
this case: right-to-left). So I would have expected the colon to appear
at the other end of the line, like so:

:The date is
2006 YLUJ 17

Moz/FF behave as expected if the second line starts with some
right-to-left text. For example:

<div dir="rtl">
The date is:<br>
2006 YLUJ 17 YADNOM
</div>

is displayed:
:The date is
2006 YLUJ 17 YADNOM


Do Moz/FF wrongly consider the digit at the beginning of the second
line as a strong character, giving the colon the same (left-to-right)
direction? Or am I missing something else?

Thanks

Patrick