The best replacement for ipfw is iptables.

The best alternatives to ipfw are rated as following:
★★★★★  iptables
★★★★★  pf
★★  south dakota state
★★  iupui
★  ipf
★  ipfilter
  western illinois
  north dakota st
  ipfw
  southern utah

Your alternative for ipfw is not listed? Suggest!
Speaking about ipfw, one probably shouldn't forget about altq.

Other search terms related to ipfw and their ratings are:
★★★★★ altq
You have in mind a companion for ipfw and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of ipfw in all years. Click on graph for more.
ipfw
iptables
pf