If you're an add-on developer you already know that the XPI format used by Firefox is simply a ZIP file with a different extension. While developing you probably get sick of creating an archive (or zip file), changing the extension to XPI and then double-clicking the file to load the extension into Firefox. In and of itself, it does not take too long to do this, seconds at most. But, while in the fury of development, this "few seconds" adds up.