I'm having a different experience.

  Aquarium: 60fps until 20k fish, where I hit 50fps. 30k at 34fps
  Blobs: maxed out resolution and number of blobs, still 60fps
  Field: 60fps at "lots"
  Fishtank: 60fps with 1k fish and sharks
  Spacerocks: 60fps on lots
  Sprites: 60fps on 10k

System: - FF 140.0.4 - Kernel: 6.12.37 - CPU: AMD Ryzen 9 5900X - NVIDIA 4080S (575.64) - 186 tabs open (mostly YouTube. >20 active)

I get a bit worse on my M2 Macbook Air (128 tabs), but pretty close results.

Maybe you need to open more tabs?