Recently I had a package disappear on me from the mainline repos but one of my apps had a hard dependency on that version. I managed to find a machine that still had that package installed so here's what I did.
Get the file list:
dpkg -L firefox-esr|sed 1d|grep -v "package diverts" > esrfiles.txt
Strip the list down to only the actual files:
{ for i in `cat esrfiles.txt`; do [[ -f "$i" ]] && echo $i ; done; } > esrfiles2.txt
Use tar to get the files in their folder locations into another prefix:
mkdir pkg
tar -czf - `cat esrfiles2.txt` | tar -xzf - -C pkg
Then grab the package scripts (hopefully they haven't been cleared out by some clean operation):
mkdir pkg/DEBIAN
cp /var/lib/dpkg/info/firefox-esr.* pkg/DEBIAN/
Find the package control text in the /var/lib/dpkg/status and copy it into a file called pkg/DEBIAN/control.
You will then need to remove the Status: line from the copied text in the control file.
Rename the files to remove the package name:
cd pkg/DEBIAN
mv firefox-esr.postinst postinst
# etc....
Then you can build the package the quick and dirty way:
#dpkg-deb -b pkg
ian pkg