Can only be hardware differences between the two systems ( memory,
graphics card, BIOS settings etc... ) but has your son's system ever
been able to play flash ?

Last I heard was FF was going to drop their flash. You know any thing about
that?

Sometimes things are only temporarily out of whack. This happens when you
do an upgrade and then do not re-start the browser or when you do an
upgrade and things are not evenly upgraded. If you do a second one a short
time later it will often fix flash problems. If they persist then the
problem is more serious and I would remove and re-install Flash.

I had a chance to work on my sons computer today. I started by
installing Gnash and some other packages associated with gnash and it
started working.

Since Gnash is just another flash media player it would suggest the
problem was with Adobe Flash ( nothing new there then ! ) and may be
just the amount of graphics RAM allocated. Gnash is lighter on RAM
requirements.

What's he using to play Flash?

I just subscribed to a digital version of Sky and Telescope. Needs Flash. I have
flash, but the mag told me I didn't. I happened to be using Firefox ESR. 10.7

Switched to Iron (Chrome derivative browser which does its own flash,) and
magazine opened. Opera also opened it.

I've had the same problem with FX on Windows XP. Could be the ESR version.

I'm using 11.04. Any upgrades will have to wait until after holidays.
I'm too busy to deal with Ubuntu right now. FX ESR only gets security updates -
no other "improvements"

He is using the flash that is in the Xubuntu restricted extras. If this
was the wrong flash, it wouldn't work on my computer.

does he have flash loaded?............

Yes, otherwise it wouldn't play on my computer.

Same ISP? Same motherboard, CPU, RAM and VGA card, etc?

No, unfortunately. The ISP is the same but everythinhg is different.

Have you installed flash or the restricted extras? By default, Ubuntu 12.04
does not handle flash. Go to the software center and do a search on 'adobe
flash'.

Xubuntu restricted extras were installed. Flash is installed.

So the only difference is the hardware and something there is going to
put the finger on this - list the two system hardware specs and we'll
see what might be the problem.