>Why does the extension on Firefox block AV1 by default? It seems unrelated to the purpose of the extension.

Because AV1's decoding is slow and on some low-end hardware it makes the animation stutter.

> Also have you considered coloring the spectrum according to the musical note? So every C is red, every E is green, every G is cyan, etc.

The color is used for left-right channel separation.

cnlohr's colorchord does something similar to what you mentioned.

https://github.com/cnlohr/colorchord