I'm not sure how you get the distribution info from the command line but no doubt you don't want to delve into the internal workings of Linux to that sort of depth just now, so...
Go to the start menu icon at the bottom left (the Ubuntu logo) then go to <system> <Administration> and select System Monitor. Under the System monitor tab it should telll you which System you are running, possibly:
Release 10.04 (lucid)
Kernel Linux 2.6.32-55-generic
Something like that. (That's what my EEPC netbook's running)
Does the firefox browser not have an update link? Linux usually will have the Update manager tell you you have a load of things which need updated; libraries, applications including Firefox and new kernels. The update manager will be in the "task bar" when it wants to let you know there are updates available. Click on it, it'll ask for your password to proceed, that's the one you used to log into your account.
As for AVAST, which my wife uses on her Windows machine... You don't need an antivirus program in Linux as most viruses are written for Windows. Linux is not suceptable to Windows malware and viruses are extremely rare. (This is probably one of the reasons you are noticing a speed improvement in your machine; no antivirus overhead!
Anyway... Welcome to Linux Questions and have a great time!