I made a WebExtension! It’s called Transmitter for Transmission. It lets me add torrents to the transmission-daemon instance running on my home server. And watch their status. I think Transmission actually allows you to expose the remote API in desktop versions as well.
tl;dr on WebExtensions: it’s Chrome’s extension API, but with Promises and it’s a W3C standard. Fully supported in Firefox. Chrome/Opera need a tiny polyfill for the Promise support. MS Edge can be supported too.
Publishing experience: Chrome’s store asks for $5 to get your stuff published (one time payment for up to 20 extensions) and there’s no pre-moderation. Addons.Mozilla is pre-moderated but the extension shows up on its URL before approval. Opera is pre-moderated and the extension doesn’t show up before it’s moderated.