This add-on injects JavaScript into web pages. The addons.mozilla.org domain disallows this operation, so this add-on will not work properly when it's run on pages in the addons.mozilla.org domain.

Replaces words that describe an emoji with the emoji itself. This runs as a content script and scans web pages, looking for text that can be replaced with emoji. As an example, after installing visit https://mozilla.org and notice that the text "firefox" should change.

A good example for beginners that can be used as a "make your first add-on" tutorial and / or referenced to create other add-ons.