A lot of updates are for the sake of security. Things like patching holes
from known exploits, etc. So I would personally consider doing at least
security updates. As far as the firefox issue, you'll likely need a
bootable CD or thumbdrive for install (see "FWIW" below)... so you can just
check the Firefox version ahead of time when running it live. I've never
seen default Firefox to update a version through one of cannonicals
updates.

FWIW, Upgrading to a new ubuntu version through the "update" center can
cause some issues. It is an unfortunate but true problem. I am not saying
it always causes problems, just that it does sometimes cause problems. Plus
it will be easier to get set with a new home partition on a fresh install -
so there is some other reasons to do it fresh besides the fact that
cannonical intentionally sabotages their product every six months! :-D. As
long as you back home up first and copy it into your new home partition, all
should be well.