How do I uninstall a backport?

[Noel O'Boyle <no228@cam.ac.uk>]

Dear all,

I installed the backport of firefox 1.5 from backports.org. I have had
some problems (creating new bookmarks, and with security certificates)
and decided to uninstall it. Now I find that I cannot get back the old
Debian stable firefox 1.1.

I commented out the following line in /etc/apt/sources.list:
# deb http://www.backports.org/debian/ sarge-backports main

and removed all of the lines I had added to /etc/apt/preferences (which
restricted the updates to firefox).

Then I tried to "apt-get update" and "apt-get upgrade firefox", but it
said I already had the latest version of firefox. So I removed it
"apt-get remove" and then tried to install it, but I got:

Reading package lists... Done
Building dependency tree... Done
Package firefox is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package firefox has no installation candidate

I think I've messed my system up. Can anyone help?

Regards,
 Noel