Gnome System Manager Output
 

Linux: Is it normal for my Gnome System Manager to always indicate that I'm sending out an average of 500 bytes/sec even when my browser (Firefox) is completely closed.

Depends on what services you are running. Try running nethogs, a program that displays which service use how much bandwidth. Then you can determine it it's a problem or not:)

http://nethogs.sourceforge.net/

Thanks, I wasn't even aware of nethogs. J.Bin