Go to Samfirmware, use the pulldown boxes to find a GB stock rom for your carrier/country (the other sites link back to Samfirmware's files in almost all cases), download it, flash it with Odin. Tho you may have trouble getting 2.3 - that's ancient.
Also, if you're returning the phone for a warranty claim, you do not need to go back to 2.3, irrespective of what you may think or what you may have been told. You simply need to make sure you have any rom for your carrier/country on your phone (you could flash the latest JB rom released by your carrier, for example), your flash counter is reset, do a factory reset once you've flashed the rom just in case, and send it off.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Most retarded thread on XDA in 2013