There is NOT ferret specific info in relation to Filpronal in what I
found but as you can see it does appear that the assertion that this
compound has toxic effects in ferrets does *NOT* seem to have a basis.
If the person making the claim to you could provide some references
instead of simply stating that such proof exists that would help in
looking up this compound.  Frontline has been the victim of rumors
without fact behind them before, and these assertions you have read
could be more of the same.  I don't know.  If there is new hard data
in either direction I'd love to know about it.
 
These will give you a feeling for part of what I found and why I am not
afraid of the compound at this point:
 
Filpronal can lengthen the estrus cycle in rats, but don't know if it
does that to members of Carnivora.
 
Reproductive adverse effects of fipronil in Wistar rats.
Ohi M, Dalsenter PR, Andrade AJ, Nascimento AJ
Toxicol Lett. 2004 Jan 15; 146(2): 121-7
 
For reptiles it can be toxic -- but it ia huge leap from reptiles to
ferrets:
 
Toxicity and pathogenicity of Metarhizium anisopliae var. acridum
(Deuteromycotina, Hyphomycetes) and fipronil to the fringe-toed lizard
Acanthodactylus dumerili (Squamata: Lacertidae).
Peveling R, Demba SA
Environ Toxicol Chem. 2003 Jul ; 22(7): 1437-47
 
"Reptiles in arid and semiarid zones are frequently exposed to
insecticides sprayed to control locusts and grasshoppers... The high
toxicity of fipronil to lizards was not previously known, suggesting
that follow-up studies (e.g., subacute dietary tests) are needed to
provide adequate data for risk assessment."
 
Here's an interesting thing:
 
A case of accidental ingestion of ant bait containing fipronil.
Fung HT, Chan KK, Ching WM, Kam CW
J Toxicol Clin Toxicol. 2003 ; 41(3): 245-8
 
"A 77-year-old woman accidentally ingested a commercial ant bait
containing fipronil without development of obvious toxicity, supporting
the safety of this new insecticide as demonstrated in animal studies.
However, concentrated agricultural products may be more toxic..."
 
Fipronil: environmental fate, ecotoxicology, and human health concerns.
Tingle CC, Rother JA, Dewhurst CF, Lauer S, King WJ
Rev Environ Contam Toxicol. 2003 ; 176(): 1-66
 
That article presents the concern that it might bioaccumulate in the
tissues of fish.  Also mentions that some ground nesting birds do suffer
toxic effects when exposed to the high levels used in some types of
agriculture, but waterfowl for some reason do not.
 
Toxicology of newer pesticides for use in dogs and cats.
Hovda LR, Hooser SB
Vet Clin North Am Small Anim Pract. 2002 Mar ; 32(2): 455-67
 
"The past 10 years have witnessed the development of several new
insecticides that have been specifically designed to exploit physiologic
differences between insects and mammals.  This has resulted in products
that seem to have a wide margin of safety when used in dogs and cats.
Compared with the more acutely toxic organophosphorous, carbamate, and
heavy metal insecticides as well as with the environmental problems of
bioaccumulation associated with some of the organochlorine insecticides,
these newer insecticides such as fipronil, imidacloprid, selamectin,
lufenuron, and nitenpyram seem to alleviate these known problems while
still providing satisfactory insecticidal activity."
 
Now, it DOES work as a neurotoxin, but as a neurotoxin which is
specifically targeted to insect neurologist which DOES differ from
mammal neurology.
 
I'm going to stop looking but anyone is free to search in Hubmed or
elsewhere to know more.
[Posted in FML issue 4396]