Look for pillows that are easy to add stuffing to, and buy extra stuffing. All stuffing will flatten over time, but you can add more when that happens to extend the life of your pillow. Since you say you like firm, I would lean toward foam filling. Down (feathers) tend to break apart more easily than synthetics.

Fluff your pillows. Fluffing your pillow in the morning can allow the stuffing to expand once more and prolong the life of your pillow.

Use a good pillow case and wash regularly to prevent body oils from damaging the filling.

You mentioned liking firm pillows, and that they don't tend to last long. I would guess from that that you are a side sleeper who holds your pillow when you are asleep. You might could look into cylindrical pillows that better fit this sleeping position. If you fold a pillow in half to make it fit, then you will end up crushing the fibers and reducing its functional life. As an example, this is a firm foam, solid roll, foam pillow with a bamboo cover available on Amazon.

https://www.amazon.com/AllSett-Health-Cervical-Removable-Ergonomically/dp/B07PJL9W6D/