installing packages from testing in sarge

[Martin Paraskevov <ze.marto@gmail.com>]

Hello,

I am a newcomer to the list and have the following question to ask. I
have recently installed debian sarge (the stable distribution) but when
I search for mozilla-firefox with:

apt-cache show mozilla-firefox

I get an older version of the browser.

1. Can I install software from the testing distribution under
sarge.  For example if I add a line in the /etc/apt/sources.list
file. Or do I have to upgrade to the testing etch.

2. What command can I use in the shell to edit the
/etc/apt/sources.list file but not manually but through interface. I
think there was a tool like this but I don't remember what it was.

Regards,
Martin