Browser extensions are composed of Javascript, HTML, CSS, and images. Before installing any extension I will usually review the source code. This is important to verify that an extension is not doing anything malicious. Luckily, viewing the source code is simple if you use the CRX Viewer. (source)

Browse to the extension install page and copy the URL of the extension. (example) Go to https://robwu.nl/crxviewer/ and paste in the URL. (example) You can review the code in your browser or download all files and review in your text editor.

You can also install the CRX Viewer extension to quickly review any extension from the store page.

Thanks to Rob for this great tool!