Resolve "Editors are slow on Firefox 52"

Closes #235 (closed)

This turned out to be because setAttribute was used to update DOM elements whereas this should be done through normal JS properties. The property that was set was the tooltip of the icon, which only changes when the first character is entered.

From the reference:

Using setAttribute() to modify certain attributes, most notably value in XUL, works inconsistently, as the attribute specifies the default value. To access or modify the current values, you should use the properties. For example, use Element.value instead of Element.setAttribute().