As a developer, I'd really like if I could publish an extension as a single archive file and have users load it into whatever (compliant) browser they like - however, the standard doesn't seem to actually support this. When it comes to packaging, there is nothing more than a vague acknowledgment:

Browser extensions MAY be distributed and subsequently loaded as individual files or via a container, such as a .zip archive. Regardless of the packaging format, these containers MUST contain the following: [manifest.json, Necessary JavaScript files (<filename>.js)] [...]

Browser vendors MAY require that extension packages include a digital signature. The signature MAY indicate the source of the extension, such as a distribution store or the extension's publisher. This aspect of packaging varies by browser, and browser implementers MAY enforce different requirements.

I guess that's a concesson to all the politics surrounding app stores, but it's disappointing nevertheless.