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 addons.mozilla.org or chrome.google.com 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.