bug 1058698 - async ContentWorker

> That isn't great especially when this is only to support testing

fair enough.
> for every add-on we add a sync-call for every tab opened.

no, `tab-events.js` is a singleton (per tab), loaded only once for all addons (and only if there are SDK addons at all) via the `FramescriptHelper.jsm`.

(but now that it does more than just events, maybe a better name is in order, like `sdk-tab-helper.js`?)
> this starts to fail if two different add-ons using different paths are installed at the same time!

yes, though (i think) only if they also change the `LoaderHelper.jsm` code, which seems an unlikely scenario (outside our tests).