Jens,
As far as I'm aware, there has been no changes in 1.91 that would affect what you are trying to do. All you need to do is first command the unit to point his weapon at the target, using the "unit dowatch target command. You can either watch a unit, or you can watch a position (in [x,y,z] format). That command will get the unit to look straight at the target. Then, use the unit fire "weapon" command. This will get him to fire his weapon once directly at where he is pointed.
I think the dofire command will only work when you are trying to fire on a living unit, so that may be why you haven't been able to get this to work. Now, keep in mind that, while using the "dofire" command will get the unit to AIM his weapon at the target, and try to compensate for things like distance and elevation so that he actually HITS the target, the above method does none of these things. Basically, using the "dowatch"/"fire" command won't always give you an accurate shot, especially at longer ranges or weird angles.
Hope that helps.