Currently, the UI assumes that when an addon is installed, it uses a AddonInstall object to do so (and its relevant events). However, this isn't neccisarily the case - a provider should be able to instantly install a new addon (and uninstall one), using only the onInstalling/onInstalled and onUninstalling/onUninstalled events.
This is needed to properly support the search-engine and plugins providers.
That looks more like we can only have automated tests here.
Landed on trunk as a part of bug 554007
Verified fixed based on check-in. A possible automated test is still outstanding.