This thing is conceptually quite complicated with so many parts: background script, content script, UI for button in tool bar or in address bar, pop-ups, options pages and permissions. Also I didn't set up a nice dev environment. Where the hell was the diagnostic output from my extension?
Anyway finally I beat Tuji to it in publishing a webextension. I had a very straight-forward and specific need, so I just quickly coded a background script, put up a simple button to toggle its on-off status and nabbed a picture from Tuji's Dropbox as icon. OK. I did a little bit more work like using uuidgen to get a uuid for the extension.
I don't think others will have a need for this extension.