The optimization. Usually built with -O3, maximum optimization, the plain r3 version. People asked for other versions to test, the r3_O2 version is built with -O2, highly optimized but often safer (stabler) and the r3_s version is built with -Os, optimized for size, then speed, perhaps best on memory restrained systems.
As I said, they were sort of experimental as someone questioned my using -O3 so I built the others for comparison. The seamonkey-2.42.9esr_r3.en-US.os2.zip is probably the best.