Wouldn't it be better to learn TB3 instead of regressing? Even if you can
install it, how long before you upgrade and the upgrade messes up your TB2?
Eventually TB2 will have to go. It is just a matter of time. You can install
TB2 locally to your home folder and then move it to your root partition
(/opt) by file copying as root. It is just a matter of finding the same
files by doing a search then writing down the locations and then starting a
nautilus admin session using gksu nautilus. If you can find a TB2 deb then
you don't even need to do that. You might find a deb in Debian repos as it
runs behind the times significantly... all in the name of stability of
course.

ubuntu-tutorials.com/.../manually-install-thunderbird-2-ubuntu-704\
/

As for your filters, how did you move to 10.04? Was it by upgrade or fresh
installation? If it was by fresh installation did you save your old
.thunderbird folder? If it was by upgrade, then your filters may still be
there in another folder. Sometimes you get a second identity. Look in the
.thunderbird folder and see if there is more than one identity. It will look
like ikthk8v.default. Then you can copy the files from the old to the new
and delete the old identity or better still you can change identities in TB
bird's settings under properties. This may not be the case and you may be
using the old indentity and it has overwritten the previous one, but that
would be strange. It has never happened to me. I still have the same inbox
and filters that I have had for five years or more and I do a fresh
installation every six months.