With my set of tabs that Firefox start every time:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
11982 webbrows  15   0  243m  86m  25m R 16.0  8.6   0:16.25 firefox-bin

Take Gmail out from the list:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
12038 webbrows  15   0  196m  63m  22m S 19.9  6.3   0:09.86 firefox-bin

Take Google Portfolio out too:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
12080 webbrows  15   0  173m  58m  21m S  4.3  5.7   0:09.86 firefox-bin

Welcome to Web 2.0!

Now I use Gmail Notifier instead of leaving the tab open all the time, since I rarely get new emails from my Gmail account anyway.