Thats not quite right. If you add up the -/+ buffers/cache used and free you'll find they add up to the total mem. The free mem is simply the -/+ buffers/cache free value.
Just had to mention that to calculated the amount of ram taken up you have to use the -/+ buffers/cache used mem from the total mem.
This gives approx. 45% mem before firefox and 64% with firefox, which is great if you ask me. Lots of potential.