Hello,
I have more than one display, and I prefer not to run Xinerama.  One 
display is 30", the other 17", so there's a pretty large blank spot with 
Xinerama.  Instead, I just run old school multi-head.
It's rare that I want to run a program on both displays.  Emacs has it 
right in the menu: "open frame on display".   Many other programs will 
happily open more than one instance.
However, firefox throws up a dialog box claiming that it's already 
running but not responding.  I want to do something like this:
DISPLAY=0.0 firefox &
DISPLAY=0.1 firefox &
and not get an error message.
In the process of typing this message, I've figured out two solutions:
1. just run Konqueror in the other display
2. create a new firefox profile using firefox --profilemanager, and then 
start firefox on the other display with "firefox -P other-profile 
-no-remote"
I knew google would have the answer, I just had to figure out what to 
search for.
This message is posted anyways in case anybody else finds it useful.
P.S.  I highly recommend 30" displays.  It's like multi-head, but 
better.  Some hints.
1) treat it like multi-head.  Sit as close as you would to a normal 
monitor.  This is unnatural because it exceeds your field of view 
substantially, but your windows will be small enough that they fit.
2) use black backgrounds.   30" of screen is a lot of light.  Reduce 
your eye strain!
3) Watch for sales: dell.ca often has some awesome one day sales.  My 
brother bought his 3007WFP for $1200.
4) You need a dual-link capable DVI connector.  Most DVI connectors are 
single-link, although it's a lot more common now then it was 2 years ago.