Avoid DOM Mutation events

[arantius]

Mozilla review of 0.9.9 said:

This version didn't pass full review because of the following issues ... you need to correct them to get full approval. Thanks.

2. DOM mutation events profoundly degrade performance, and that degradation is not reversed by removing the listeners. We can't allow mutation listeners except when they're absolutely necessary, and never for the top-level browser document.