As part of figuring out how successful hidden tabs are, we should add telemetry to count the number of tabs that are being hidden by the users. If there's any questions about this, :bsilverberg has been doing this for other parts of the WebExtension framework and might be able to help.

I think this should be done in tabbrowser where stuff is hidden/shown, rather than in the api, but I suppose it doesn't matter (unless anything internal hides/unhides).