beastify

What it does

The extension includes:

When the user clicks the browser action button, the popup is shown, enabling the user to choose one of three beasts.

When it is shown, the popup injects a content script into the current page.

When the user chooses a beast, the extension sends the content script a message containing the name of the chosen beast.

When the content script receives this message, it replaces the current page content with an image of the chosen beast.

When the user clicks the reset button, the page reloads, and reverts to its original form.

Note that:

if the user reloads the tab, or switches tabs, while the popup is open, then the popup won't be able to beastify the page any more (because the content script was injected into the original tab).
by default tabs.executeScript() injects the script only when the web page and its resources have finished loading. This means that clicks in the popup will have no effect until the page has finished loading.
it's not possible to inject content scripts into certain pages, including privileged browser pages like "about:debugging" and the addons.mozilla.org website. If the user clicks the beastify icon when such a page is loaded into the active tab, the popup displays an error message.