I currently have 203 tabs open, and firefox has been running for 13 days. It's using 2.1gb of memory. I churn through tabs a lot. Firefox used to have memory problems, but it hasn't been a problem for me in at least a year. 10mb per tab is pretty good, IMO.

Interesting, because I have 5 tabs open (running the latest version of FF), and it's using over half a gig of RAM. It's been running since monday.

Perhaps I need to take another look at my addons, but I only have four (firebug, web developer, ad block, and diigo), so I really doubt they are the cause.

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.