ESR is typically used by schools, corporations, etc. that don't want to execute a full validation process against functional releases every 6 weeks, which is the current FF rapid release schedule. Mozilla took one baseline release - v10 iirc, and will apply only the security patches from each incremental release and roll them out as an ESR dot release.

So ESR users only get functionality updates once a year, which is the planned cycle for full ESR releases. If you're on ESR 10.0.5 you are running FF10 with the security patches of the releases since. You'll get no new functionality until what is planned to be ESR 17, which will be based on FF17 in 5-6 months.

The official Mozilla description is here - ESR Overview

I agree that running old software is a bad idea - 11 releases worth of security updates have been made since FF5. Not to mention the start-up acceleration in FF6, the 50% memory usage reduction in FF7, SPDY inclusion, javascript performance increases, etc. Your choice though.