Kill, destroyed and taking a large percentage are all not representative of what Firefox did to IE, so it all comes down to how small the "part of" can be, I guess. I'm not a native speaker, so it might be that I sized it incorrectly.

The origin is pretty interesting.

> Decimation (Latin: decimatio; decem = "ten") was a form of Roman military discipline in which every tenth man in a group was executed by his cohorts.