Extension Source Viewer is a free Firefox add-on which can display the source code of other Firefox and Chrome extensions.

Visit a compatible page - usually an extension's or home - and a crx icon appears. Clicking this displays options to download the add-on as a zip file, without installing it, or view its source.

The Viewer option displays a simple list of everything in the source, including HTML, JS, CSS and image files, and clicking any of these displays it in the preview pane.

A search box with regular expression support helps locate text of interest.

You can also save content as usual from the right-click menu, handy if you'd like to save a graphic from the extension without installing it first.

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If you're curious how a browser extension works, Extension Source Viewer could offer some clues, and it also comes in handy if you'd like to save icons or other images without installing it first.