ClearInput от Eric, KJ7LNW

This add-on places a "(X)" symbol on each visible input box to clear the input when clicked. For example, search inputs show a "clear search" button on the searches, and this applies the icon to all input fields. Other browsers support an automatic "Clear search" icon on <input type=search> inputs fields, but at the moment, Firefox does not. This plugin adds that icon for <input type=search> and <input type=text> if the input box is visible at page load.

For X-Windows users (Linux, etc), you may not wish to accidentally select content of a text field, because it will affect your middle-click-paste feature (a.k.a X-Windows cut buffer), which would replace the selection. On Linux systems, it is a common work flow is the clear an input box and middle-click to paste, but unfortunately selecting the text to replace it also modifies the cut buffer. Thus, a "clear input" button solves that, such that clicking the "(X)" will clear the input so middle-clicking the input will paste your cut buffer.