As for sandboxing, I don’t think we’ll be sandboxing much, which yes does give rise to the potential for the add-on hook to do something to the extension manager, however I think we can be quite careful and restrict that if we need to. Perhaps by putting locks in place so the various API calls that can install/uninstall extensions cannot be used during the call to the hook. Perhaps even going so far as saving the state of the extensions database and restoring it afterwards.

I’m hoping that we won’t need to go too far though. Ultimately if an add-on wants to cause problems then it has adequate time to do it while installed, I’m not sure that we would have a particular problem during uninstall, and likely the only way to stop such problems would be a complete sandbox as you say, making the feature essentially useless.