Normal taxi times are about 12 minutes for both takeoff and landing. Let's use 15 mins for an example. Each engine burns approx 600 lbs / hr at idle and requires a 3 minute cool down (or 3-5 minute warm up). So, that's 12 minutes of SE taxi (saving 240 lbs of fuel) or about half of what you're claiming. Yes it helps but fuel used for taxi is about 1% of the total consumed so it just doesn't add up to that much as compared to other things. TBH, if we REALLY wanted to save money, we'd fly at green dot (and w/ the significantly over blocked leg times, we'd still be on-time mostly). That would save more money by far.

I have a better solution. Why don't we INVEST in better people at the gate, more friendly ticket counter personnel, update our flyfrontier app and pay labor more. Then, we improve our on time rating, our customer service and our reputation. Then we charge more for a ticket to make up for the invested funds. This is the same way ALL of the other profitable airlines did it.