A complete 3.5 update for Hell in Freeport has yet to be done, but the following pieces have been updated:
Monsters: See the 3.5 Monster Manual for updates of those monsters. (See also note to p. 5, below.) See The Book of Fiends for 3.5 versions of monsters from Legions of Hell: bulugon (p. 151), enforcer of Dis (158), faceless (160), ice stalker (170), Jalie Squarefoot (171), spinder (190), vierhaander (192).
Prestige Classes: See Freeport Classes (3.5 Revisions) for 3.5 revisions of the prestige classes from Focus on Freeport #9 and 10. See The Book of Fiends for the 3.5 mountebank.
p. 5, Encounter 1: Devil on the Docks:
I've just started preparing for running Hell in Freeport and noticed the first encounter with Cornugon has completely changed in 3.5 since it is not a CR 10 anymore and is a CR _16_. Thankfully I noticed this before running it. That was a TPK waiting to happen. Has anyone dealt with this? [jamesbraun98, 2/9/2004, "Hell Port 3.5"]
Many fiends got power-boosts in 3.5, so you will probably want to make changes to many of the encounters in HiF. Either reduce the number of devils in those groups, or swap them for another devil type of the proper CR.
...or just wait until your players are higher level! ['Dr. Nuncheon', 2/28/2004, "Re: Hell Port 3.5"]
Unfortunately, the MM doesn't appear to have a satisfying answer for the Firebrand Killer. A bone devil is about the right power level (CR 9; add 2 HD to make it CR 10), and can fly at will, but lacks the powerful attack spells that define that encounter.
Here's one suggestion that preserves the spirit of the encounter, if far from the letter: a Half-Fiend Efreet (CR 8, +2 for template = 10). A flying hellspawn with fire magic and a overwhelming hatred for servitude? Sounds like the Firebrand Killer to me.
I mentioned the changing CR problem to Kevin Marsh (a friend who also GMs), and he had the following suggestion, as yet another tactic: Just use the 3.0 stat blocks as given in the module, but change the devils' DR to match the 3.5 system. That's the least work for you, and in most cases, it won't be *that* far off of the 3.5 versions. If the PCs (and players) have had limited experience with 3.0/3.5 devils, they may not even notice the difference.
p. 84, Wycleffe: The sample mountebank is updated in The Book of Fiends (pp. 199-200).
pp. 85-86, Automaton: See 3rd Era Freeport Companion, pp. 130-131, Infernal Automaton.
p. 86, Pit-Brier: See 3rd Era Freeport Companion, pp. 131-132.