There are modules in the netfilter distribution called ipchains.o and ipfwadm.o. Insert one of these in your kernel (NOTE: they are incompatible with ip_tables.o!). Then you can use ipchains or ipfwadm just like the good old days.
This will be supported for some time yet. I think a reasonable formula is 2 * [notice of replacement - initial stable release], beyond the date that a stable release of the replacement is available.
This means that for ipfwadm, the end of support is:
2 * [October 1997 (2.1.102 release) - March 1995 (ipfwadm 1.0)]
+ January 1999 (2.2.0 release)
= March 2004.
This means that for ipchains, the end of support is (FIXME: get real dates):
2 * [August 1999 (2.3.15 release) - October 1997 (2.2.0 release)]
+ July 2000 (2.4.0 release?)
= March 2004.
So you don't have to worry until 2004.