Moth balls!
Had a massive flea attack with our dog. Vet said “Boric acid”.
$5 a container (a pound?) at local hardware store. Sprinkled on carpet, brushed into the pile of the carpet, to get down deep where the little bastards hide, with a push broom. You’ll have boric acid dust on your furniture just from applying, as the powder is so fine; so you’ll have a little cleanup to do.
Bathed the dog every few days for a couple weeks to get the live hitchikers off them; quit using Frontline and switched to K9 Advantix (not sure if there is an equivalent for cats). After a few weeks the fleas were history and we have not had a problem since (3+ years); we reapplied boric acid annually, as we figured vacuuming eventually sucks up most of it over time.
Boric acid dessicates the fleas and as the eggs hatch (did you know they can “live” as eggs for more than a year) they get dessicated too.
Good luck!