Seconded. The third-party sites are better with the data handling. OTOH, the best fares are often promo-code-sale ones bookable only on Frontier's site, with the difference showing up the most on longer hauls. If you can get an 80% or 90% for your dates it'll probably be lowest. OTOH, it won't ACTUALLY be 80% or 90% because they pad in junk fees (plus actual taxes) that aren't subject to the discount.

For shorter hauls like ATL-Florida that sometimes dip to $29 or thereabouts each way, the promo code sales take less off and the best savings come from buying the ticket at an airport.