Fireflies are fascinating and beautiful creatures. Another user mentioned donating to xerces society which is a great idea. Another thing that you can do if you are fortunate enough to own land is to replace your lawn (non-native, mowed grass) with native plants and leave the fallen leaves undisturbed.
I replaced my entire lawn with species native to my region and my entire yard is currently covered in multiple species of fireflies. It's so magical to see all the different colors and flashing patterns. Mind you, I live in the city so it's only my yard that is really providing for them. All my neighbors' yards have either no fireflies or a few.
You don't have to replace your entire lawn like I did. Just setting aside some space for our wildlife neighbors is better than nothing. Remember plants are the foundation of almost every food web. For me, it's magical to go outside and see the new blooms and growth, look for new creatures that show up, and just walk the little paths in my small yard. In an age of ecologic collapse and climate change it gives me some sense that I can have a measurable, positive impact and that really helps me mentally.