O.K. To partially answer my own question I found this in about:config
mousewheel.with_shift.action
However this is one of those enumerated keys, and I couldn't find a
description of the available values on line. In any case, the default
action of 2 was not what I wanted, so I played around with other values.
Unfortunately all of the other values use the mouse for vertical
scrolling, not horizontal. For now I set it to 0, which only scrolls
a little bit, and at least leaves me on the same page.
Another point is that on the old PC I'm using FireFox 12.0, since that's
apparently the newest release that supports Windows 2000. So in the
about:config I don't see some values available in the latest versions
like:
mousewheel.with_shift.delta_multiplier_x
mousewheel.with_shift.delta_multiplier_y
mousewheel.with_shift.delta_multiplier_z
--
Gabor