Lawrence's Ambient Fireflies is a data pack, not a resource pack. It belongs inside a world's own folder, not in the resourcepacks folder.
Adding it to a new or existing world
New world:
1. On the Create New World screen, open the More tab and click Data Packs.
2. Drag the pack file onto the window, or click Open Pack Folder and drop it in.
3. Move the pack from the Available column to the Selected column, then create the world.
Existing world:
1. Close the world.
2. From the singleplayer list, select the world and click Edit, then Open World Folder.
3. Open the datapacks folder inside the world folder and place the pack file there.
4. Load the world again.
Checking that it loaded
1. Open chat and run /datapack list.
2. Look for the fireflies pack under the enabled packs.
3. If it is listed as available but not enabled, run /datapack enable followed by the pack's name. Use tab-completion to fill in the exact name.
4. If you added the file while the world was open, run /reload to pick it up.
You need cheats or operator permission to use these commands. If your world has cheats off, you can open it to LAN with cheats allowed for the current session.
The pack leaves vanilla gameplay intact, so adding it to a long-running survival world won't change your mobs, items or terrain. It only adds ambient particles in swamps.