As confirmed by Mozilla developers, the webextensions technology have limited features to ensure security of the browser. So, they will not implement the API required for 'double-click on tab' feature. Look here(https://bugzilla.mozilla.org/show_bug.cgi?id=1246706) and here(https://bugzilla.mozilla.org/show_bug.cgi?format=default&id=1356949) for more details.

So, unfortunately, you should stop waiting for that feature. Meanwhile, may be the latest approach in v3.0 - long left click anywhere on the webpage to toggle - may suffice (or hopefully exceed) your needs.