*chuckles* uh, around 428 tabs on my Lenovo Thinkcentre. BUT they just accumulated over time with me keeping everything open (most just unloaded) so that I can go back to everything easily, and setting up firefox to restore previous session every time i open it. Anyways, that's totally on me, and I think that usually just uses around 3GB upon first run of firefox, until I start opening more tabs like youtube and such. Right now I'm using a Dell Optiplex with 12GB RAM with 10 tabs open (3 are just google) in Firefox and I'm seeing approximately, well, at the moment, exactly 3GB used by the entire system. Only Firefox is running.
My session manager and tab unloader extensions come in handy sometimes.