Provides an icon in the Add-on Bar. Can be hidden to reduce unintended saves.
Provides an item in the context menu only if file URI scheme.
Provides an option to limit operation to URIs matching a regular expression, to avoid unintended saves.
Provides info in Error Console, if you care to look there.
A couple nice SVG example files to try out with linked from http://srguiwiz.github.com/save-back-xpi/save-back-examples.html
Designed specifically for use with https://github.com/srguiwiz/adj-js
as illustrated http://srguiwiz.github.com/adj-js/user-docs/#example-workflow
Recently seen working for page text edited in Firefox via contenteditable="true".
Since Firefox 57 replaced with newer https://addons.mozilla.org/en-US/firefox/addon/download-serialized-dom/