Firefox Mobile adds a background when triggering :active on a input field.

Vivien reported that this is because of !important in the core content.css and therefor can't be over-written.

<style>
  *:active {
    background-color: none !important;
  }
</style>

Unfocused:

Focused:

On Android this focus is yellow, these screenshots where taken on Fennec (Mac OS X) that's why it is blue.