I'm at 2.9G virtual 1.277g resident with about 130 tabs open (yeah, I'm a goofball but I like my tabs). It's pretty responsive on a thinkpad x220 i5-2540M @ 2.6ghz, 8G of ram and an SSD.
I tried to use chrome once and it ate all my ram, and they were saying it uses less ram then firefox back then.
https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...
For me, it is /the/ absolute killer feature for browsers.
---Alex
More search tips can be found here: https://support.mozilla.org/en-US/kb/awesome-bar-search-fire...