I think that the browser footprint is fairly large, but as you open more tabs it uses proportionately less ram per tab (though it must level off at some point). I'm using 15 extensions (the only one of yours I'm not using is diigo), I doubt they make much difference.

For some time during the 2.x and 3.x series, firefox would leak memory from closed tabs -- the memory usage would keep going up and up with tab churn, and I'd have to restart the browser to get the memory footprint back down. I don't see that with v4 or v5, and I usually restart the workstation before I need to restart firefox.