alum:~# apt-cache policy iceweasel
iceweasel:
  Installed: 2.0.0.13-0etch1
  Candidate: 2.0.0.13-0etch1
  Version table:
 *** 2.0.0.13-0etch1 0
        100 /var/lib/dpkg/status
     2.0.0.12-1 0
        500 http://ftp.no.debian.org lenny/main Packages
     2.0.0.6-0etch1+lenny1 0
        500 http://security.debian.org lenny/updates/main Packages
alum:~# apt-cache policy firefox
firefox:
  Installed: (none)
  Candidate: 2.0.0.6-0etch1+lenny1
  Version table:
     2.0.0.6-0etch1+lenny1 0
        500 http://security.debian.org lenny/updates/main Packages
alum:~# apt-get -t lenny/updates install iceweasel
Reading package lists... Done
Building dependency tree
Reading state information... Done
iceweasel is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
alum:~# apt-get install firefox
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.

Since you only requested a single operation it is extremely likely that
the package is simply not installable and a bug report against
that package should be filed.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
  firefox: Depends: iceweasel (< 2.0.0.6-0etch1+lenny1.1~) but 2.0.0.13-0etch1 is to be installed
E: Broken packages