Firefox uses hardware acceleration (GPU) to make the browser faster, The whole web at maximum FPS: How WebRender gets rid of jank.
When you open things there is a lot of things the browser needs to do in order to show you what you opened, to do so quickly more resources are used.
If we made some changes that would make the GPU only "spike" to 50%, that would mean that the calculations done would take twice as long making Firefox that much slower.
Unless you notice the spike by other means, such as frame drops, then there is nothing to worry about.