Frontline plus is not indicated for rabbit and therefore my advice is to not use Frontline plus on rabbit.
There is another product, Advantage I, which can be used for rabbits. Here is some more information about the product:
For the prevention and treatment of flea infestations on cats and dogs, and for the treatment of biting lice (Trichodectes canis) on dogs.
For the treatment of flea infestations on pet rabbits.
Fleas are killed within one day following treatment. One treatment prevents further flea infestation on dogs for up to four weeks, three to four weeks on cats and up to one week on pet rabbits. The product can be used as part of a treatment strategy for flea allergy dermatitis in the cat and the dog. The treatment should be repeated after 4 weeks.
(Dogs) In case of biting louse infestation, a further veterinary examination 30 days after treatment is recommended as some animals may require a second treatment.
Administration to the Cat/Rabbit
Part the hair on the pet’s neck at the base of the skull until the skin is visible. Place the tip of the pipette on the skin and squeeze firmly several times to empty the contents directly onto the skin